The Longines Sommatore Centrale 13ZN-12

The Ultimate Guide to the Famous Double-Handed Chronograph

14/02/2024

In this article

Synopsis

For collectors, the Sommatore represents the culmination of Longines technical prowess during watchmaking’s golden age, housing what many consider as one of the greatest chronograph calibres ever produced the calibre 13ZN. Its historical significance, technical sophistication, and undeniable presence continue to make it a sought-after vintage chronograph decades after its production.

Using a modified 13ZN movement and distinguished by its innovative red central minute counter, this large watch came in two rare variants: the waterproof reference 5699 with pump pushers and the elegant reference 5161 with square pushers available in steel, yellow and rose gold variants.

This article is an in depth exploration of these two rare versions, everything from case construction to dial variations.

Introduction

Among the pantheon of great vintage chronographs from the 1940s, this exceptional reference stands as a testament to horological innovation at its finest. Known as the “Sommatore” or “Doppia Lanchetta” (nicknamed for its double hands with iconic red central minute counter), the Italian name meaning “adder” or “totaliser” perfectly captures the watch’s distinguishing design trait: its striking red central minute hand that shares the dial’s centre with the chronograph seconds hand. The defining characteristic of the Longines 13ZN-12, its central minute counter, represented far more than a mere stylistic choice. This innovation addressed a fundamental limitation of traditional chronograph design: the difficulty of reading elapsed minutes at a glance from a small subdial. For pilots operating in challenging conditions, where split-second timing decisions could prove critical, the ability to read both elapsed seconds and minutes from centrally mounted hands offered a significant operational advantage. The technical implementation required substantial modification to the base 13ZN movement architecture. The standard 13ZN, already regarded as one of the finest chronograph calibres of its era, utilised a traditional layout with the minute counter positioned in a subdial. To achieve central minute counting, Longines’ engineers developed an entirely new gear train that would allow the minute counter to share the central axis with the chronograph seconds hand. This breakthrough, protected by Swiss patent 228246 filed in 1942, required exceptional engineering prowess to overcome the mechanical challenges of mounting two independent timing functions on the same axis, transforming traditional chronograph architecture in ways that would influence watchmaking for decades. What makes these watches particularly fascinating from a collector’s perspective is their position at the intersection of technical innovation and historical circumstance. Developed during World War II and entering production as the war ended, these watches embodied Longines’ commitment to pushing chronograph boundaries while meeting the robust construction demands of professional users, particularly military aviators. Available in two rare variants, the waterproof reference 5699 with pump pushers and screw back case in steel, and the elegant reference 5161 with square pushers and snap back in steel, yellow, and rose gold, these timepieces continue to command respect for their historical significance, technical sophistication, and undeniable presence. At 39.5mm in diameter, exceptionally large for the period, reference 5699 combines every element a collector seeks: innovative functionality, robust waterproof construction, and the famous in-house movement specially modified to accommodate the central minutes function. The 13ZN-12 was part of the broader 13ZN movement family, produced and commercialised from 1936 to 1951, with a period of overlap with its successor, the 30CH, which Longines developed in 1947 as a more cost-effective alternative. Production ran from 1945 to 1949, with reference 5699 predominantly manufactured for the American market, an estimated 95% ordered by Wittnauer. These featured radium dials (order 23086) and large waterproof screw back cases, sold to Wittnauer between 1946 and 1948, with a final batch of just five pieces ordered in 1951 and invoiced in 1952. Some examples were specifically produced for US pilots, while a very small number reached other markets through agents like Ostersetzer in Milan, Italy.

In essence, this remarkable timepiece represents the culmination of all the finest technical aspects of Longines’ previous waterproof chronographs before the transition to the 30CH, establishing it as a long-considered grail chronograph. It’s a bold reimagining of how elapsed time could be displayed and read at a glance.

This article provides an in-depth exploration of both variants, examining everything from case construction to dial variations, movement analysis to collecting considerations.

Using archive materials and technical documentation, we’ll uncover the engineering brilliance and historical circumstances that made the Sommatore one of the most significant chronographs ever produced, a timepiece that continues to influence watchmaking decades after production ended.

Historical Context and Market Positioning

To fully appreciate the significance of the Sommatore, one must understand the economic and commercial landscape of the mid-1940s.

1936 marked a pivotal turning point for Swiss watchmaking in general and Longines in particular. The devaluation of the Swiss Franc in September of that year, coupled with the establishment of new commercial agreements with the United States that ended the ultra-protectionist Hawley-Smoot tariffs, fundamentally altered the dynamics of transatlantic watch trade.

It was against this backdrop that the A. Wittnauer Company, Longines’ American distributor since the 19th century, was acquired by the Hella-Deltah Company in 1936. This acquisition led to the formation of the Longines-Wittnauer Company, a partnership that would prove instrumental in establishing Longines as a dominant force in the American market. By the end of the 1930s, this relationship had become so successful that the United States represented approximately 80% of Longines’ global sales volume, a staggering concentration that would directly influence the development and distribution of the Sommatore.

The 13ZN-12’s development must also be viewed within the context of Longines’ broader chronograph program. The company had spent the previous decade refining its waterproof chronograph offerings, from the early experiments with moisture-resistant cases to the successful “Tre Tacche” of the early 1940s. Each iteration brought improvements in case sealing, pusher design, and movement protection. The Sommatore represented the apotheosis of this development cycle, incorporating every lesson learned while adding the revolutionary central minutes function.

The timing of the Sommatore’s introduction, immediately following World War II, was no coincidence. The war had accelerated technological development across numerous fields, and precision timing instruments had proven their worth in countless military applications. American military pilots, in particular, had come to rely on accurate chronographs for navigation and mission timing. Longines, through its Wittnauer partnership, was perfectly positioned to capitalize on this demand with a chronograph that offered enhanced legibility through its central minute display.

Pricing strategy reveals much about Longines’ positioning of the Sommatore. At $200 in 1945 (equivalent to approximately $3,300 today), the reference 5699 was positioned as a premium product, reflecting not just the complexity of the movement but also Longines’ confidence in the American market’s willingness to pay for superior functionality and construction. This pricing placed it at more than double the cost of contemporary offerings from competitors, establishing it firmly in the luxury tool watch category.

It’s worth briefly noting the other central-minute chronographs from the period, as Longines was not the only manufacturer experimenting with this unusual but highly functional design. One of the few comparable pieces to the 13ZN-12 “Sommatore” is the Mido Multi-Centerchrono, which also featured both chronograph seconds and minutes coaxially mounted at the centre.

Interestingly, Mido’s watch predates the Longines 13ZN-12 in terms of commercial release, first appearing around 1941, several years before Longines delivered order 23086 to Wittnauer in 1945. That said, while Mido may have been first to market with the concept, the execution was worlds apart. Where Longines developed a completely in-house solution protected by patents, Mido adapted existing ébauches from suppliers like Valjoux and Venus. This fundamental difference in approach resulted in dramatically different executions.

The Mido was a clever and economical design using modified third-party calibers, resulting in a smaller, more compact watch (around 35mm) usually found in elegant cases aimed more at professionals and civilians than military applications. Its charm lies in its simplicity and wearability, with some examples featuring telemeter scales (for measuring distance by sound) or pulsation scales (for medical use), but without the robustness or visual punch of the Longines.

The Longines 13ZN-12, by contrast, featured a far more complex and refined architecture: the central-minute function is driven by a dedicated wheel, friction-coupled under the dial, requiring an entirely separate gear train. This technical sophistication, combined with waterproof cases (ref 5699) and radium dials for pilot use, justified its premium positioning at more than double the Mido’s cost.

The Central Minutes Innovation

Swiss patent 228246, filed in 1942 and granted during the war years, details the ingenious solution. The mechanism employs a friction-coupled wheel system positioned beneath the dial, driven by the chronograph seconds wheel. This friction coupling allows the minute hand to advance incrementally while maintaining precise synchronisation with the seconds display. The engineering challenge lay not just in the mechanical implementation but in ensuring that the additional friction didn’t adversely affect the chronograph’s accuracy or power reserve.

The patent documentation, which I’ve studied extensively in the Longines archives, reveals the depth of thought that went into this design. The engineers had to consider multiple factors: the torque requirements for driving two central hands, the optimal friction coefficient for reliable minute advancement, and the durability of the coupling mechanism under extended use. The solution they developed was elegant in its apparent simplicity while being remarkably sophisticated in execution.

The visual impact of the central minutes cannot be overstated. The red-painted minute hand, with its distinctive tampered profile, created an immediately recognisable aesthetic that set the Sommatore apart from every other chronograph of its era. This wasn’t merely decorative, the contrasting colour and bold shape ensured maximum legibility under adverse conditions, including the low light levels common in aircraft cockpits of the period.

One crucial trade-off necessitated by the central minutes function was the elimination of the flyback capability. The standard 13ZN movement could be configured with the Taylor system, allowing instant reset and restart of the chronograph, a feature particularly valued by military users for successive timing operations. However, the mechanical complexity of combining central minutes with flyback functionality proved insurmountable given the space constraints and reliability requirements. This limitation, while notable, was deemed acceptable given the enhanced legibility benefits the central display provided.

From a manufacturing perspective, the central minutes function added considerable complexity to both assembly and servicing. The friction coupling required precise adjustment to ensure reliable operation without excessive drag on the movement. Too little friction and the minute hand might slip; too much and the chronograph’s accuracy would suffer. This delicate balance meant that only Longines’ most skilled watchmakers were entrusted with Sommatore assembly and service.

As with my other deep-dive articles on this site, I’ll provide a comprehensive overview of the nuances and differences between the various versions of the 13ZN-12 chronograph, offering insights that should prove valuable to anyone interested in these specific references.

Reference (cliché) 5699 order number 23086

Cliché image from Longines archive, Reference/Cliché 5699, order number 23086, stainless steel screw back, case diameter 39.5mm. Info from the cliché describes the following, ‘Stainless steel case, waterproof, round with screw back. 19mm lug width with spring bars., mineral slightly domed’.

Key Dates

Original order by Wittnauer for ref 5699, order 23086 dates to March 1945

Production at Longines was between 1945–1949

The watches were actually sold by Wittnauer between 1946–1948

One last batch of 5 ordered in 1951 and sold to Wittnauer in 1952

Ref / Cliché 5699

Reference 5699, produced under order number 23086, featuring pump pushers with a screw back (commonly known as sei tacche ‘six notches’), are the most collectable, and significant variant of the Sommatore family. This reference embodied everything Longines had learned about producing professional-grade timing instruments, wrapped in a package specifically tailored to American market demands.

At 39.5mm in diameter (excluding the crown), the 5699 was remarkably large for its era. This sizing wasn’t arbitrary, it reflected careful consideration of legibility requirements for professional use. The generous dial real estate allowed for clear marking of the various scales while maintaining optimal proportions for the central hands. When compared to typical chronographs of the period, which often measured 33-35mm, the Sommatore’s presence on the wrist was commanding.

They were predominantly shipped to the American market, but there are, however, a very small number that are exceptions to this (approximately 5% of the production). Information from the archives indicates that approximately 95% of these watches were ordered by Longines-Wittnaeur Watch Company (LWWco); however, a very small fraction were not actually bought by the US agent and therefore went to other agents such as Ostersetzer in Italy.

The case construction of reference 5699 represented the pinnacle of 1940s waterproofing technology. The monobloc case design, featuring the distinctive “sei tacche” (six notches) screw-back, provided exceptional protection against moisture infiltration. This wasn’t merely water-resistance in the modern sense but true waterproofing by the standards of the era. The case back required a special key for opening, and the precisely machined threads ensured a tight seal when properly secured.

The pump-style pushers represented another crucial element of the waterproof design. Unlike simple push-pieces that relied on friction fits, these pushers incorporated internal sealing mechanisms that maintained water resistance even during operation. The tactile feel of these pushers, firm but progressive in action became a hallmark of quality that collectors still appreciate today. The crown, measuring 8mm in diameter with 24 precisely-cut teeth, featured similar attention to sealing, utilising leather gaskets that were state-of-the-art for the period.

Those sent to the US will always have “LXW” stamped on the bridge next to the balance wheel, and the dials always have ‘Swiss’ at the bottom below 6. It is almost always the case that the dials are seen in the style below and have radium Arabic indices. The sub counter at 3 has a thicker style of font (the hour counter) and is slightly recessed.

Sommatore with mileage scale, so far only 3 examples with this dial type have been seen

Reference 5699 Specifications

Diameter 39.5mm (excluding crown)

Height 10mm (mineral excluded)

Lug Width 19.5mm

Crown 8mm diameter, 24 teeth

Case Construction Monobloc, sei tacche screw-back

Movement 13ZN-12 (modified 13ZN)

Production 1945–1949

The Dials (Dial Variations)

The dials were produced solely by Stern Frères. The proportions of the dials are 34mm in width, and 0.6mm thick. Notably, there is a slight difference in curvature between the dials made for 5699 and 5161. Due to the size and positioning of the feet, the dials are in fact interchangeable.

As translated straight from Longines ledgers, the general description for the dials in relation to ref 5699, order number 23086 is as follows…”Cadron courbés, Compteur d’heures, 11 blanc bâtons radium”, which roughly translates to metal dial, plain white colour (only 1 simple colour), radium baton-shaped digits, with an hour counter and a 60 minutes counter.

These dials can and do age completely differently based on storage and use. Whether it is a NOS or mint condition version as some of those highlighted below, or even some with heavy patina, they are beautiful and distinctive dials whilst being utilitarian, quintessentially tool watches.

Dial tips; under a loupe look for vertical brushed finish to the dial and recessed sub-counters. Familiarise yourself with the stern font and serifs used on these dials, look for defects, is the printing over the defects or below? The printing quality on authentic Stern Frères dials sets a benchmark that modern reproductions struggle to match. Under magnification, original dials reveal remarkably crisp printing with consistent ink density throughout.

This represents the most ‘common’ dial variant, featuring radium Arabic numeral indexes. The dial is characterised by an external fifth-of-a-second scale and an internal “chemin de fer” (railway track) minute scale. The surface displays a distinctive vertical brushing pattern, something to look for on original dials. This brushing wasn’t merely decorative; it helped reduce glare in bright conditions while maintaining legibility in low light.

Typically, the numerals on the scales feature serif typography (again something else to look for when authenticating original dials).

As with all vintage Longines timepieces, signs of efflorescence, minor paint losses, and surface scratches should be considered criteria of originality rather than defects. These marks commonly result from watchmaker handling and occasional impacts over time. Paint deterioration is particularly evident around the counter areas where the paint application is thinner and more susceptible to damage.

The central red-painted hand with its characteristic triangular profile functions as the chronograph minute counter (effectively serving as a third register).

The chronograph centre seconds hand is crafted from blued steel and features a design exclusive to this chronograph model, distinguished by its long, tapered central tail. The hour and minute hands are of the “baton luminous” style, finished in blued steel with radium luminescence.

The hour counter hand is also distinctive to this reference, showcasing the typical rounded leaf shape executed in blued steel.
The continuous seconds hand is likewise rendered in blued steel, featuring the characteristic Longines tailpiece (identical to the style found on the Ref. 23088 the Greenlander).

It is common to observe traces of rust and discolouration on the luminous material of original hands.

Above and below, an extremely rare variant with order number 23086, metallic matte silvered dial with radium arabic numeral, with unusual spiral scale (tachymeter) and mileage scale. Dial by Stern Frères Geneva. Three examples are currently known to the market with this type of dial, it is thought that these dials were specifically ordered by Wittnauer, most likely in a very small number.

Silver with a semi-gloss silver-grey finish, complemented by vertical brushing for a refined texture.
Tachymeter Scale: A central blue spiral helical tachometric scale enhances the dial’s complexity and functionality.
Telemeter Scale: The external scale is based on a 1-mile telemetric system, perfect for measuring distance through sound delay (e.g., thunder and lightning).
Seconds Track: Includes an internal concentric seconds track for precise timing.
Sexagesimal Scale: A concentric sexagesimal scale marks divisions down to the fifths of a second, reinforcing the chronograph’s technical precision.
Numerals: Arabic numeral indexes with radium lume for visibility in low-light conditions.
Hands: Blued steel or burnished “Baton luminous thin” hands, with radium lume matching the radium indexes.

There have also been at least 3 with this dial type that have been seen in the past, destined for a smaller cased 13zn12 which Longines were intending on putting into production but did not do so in the end. The dials have all been found in the US, along with a few others ordered by Wittnauer.

Above, a unique dial found amongst a package of NOS dials for Wittnauer in the US. This dial amongst a few others was intended for a smaller cased Sommatore. It is thought that Longines didn’t progress with the production of the reference but this is just conjecture. Years of research in the Longines archives show they planned to produce a smaller-cased Sommatore (using a modified 30ch caliber), but no physical examples have ever surfaced, only documentation referencing such a watch. We can deduce that these dials were intended for that reference due to their size at 29.46mm they unfortunately do not fit in the conventional 5699 or 5161.

There are small details that collectors look for when assessing dials for originality. . The recessed subsidiary registers at 9 and 3 o’clock should display perfect circular symmetry with sharp, well-defined edges. Note that as in all 13ZN, these registers are asymmetric to each other in their positioning on the dial. The printing within these registers, particularly the hour counter at 3 o’clock with its characteristic bold font, original dials show the printing sitting perfectly within the recessed areas without any signs of misalignment or overprinting.

Radium degradation follows predictable patterns, with the luminous material often displaying characteristic crazing, discolouration, or even complete loss in some areas. The interaction between degrading radium and the underlying lacquer can create various patina effects, from subtle ivory tones to dramatic tropical colour shifts. Each aging pattern tells a story of storage conditions, exposure to light, and decades of use. The key lies in distinguishing between attractive natural aging and damage from moisture, impact, or poor storage. Spider-web crazing in the lacquer, minor radium loss, and subtle colour shifts toward ivory or cream tones should be considered signs of authenticity rather than flaws.

The characteristic red central minute hand should display a specific shade and consistency of application. Original hands show evidence of hand-painting, with subtle brush marks visible under magnification. The blued steel of the chronograph seconds, hours, minutes, and subsidiary hands achieved their colour through thermal oxidation, creating the deep, even blue that chemical bluing cannot replicate. The “Swiss” designation at 6 o’clock on reference 5699 examples provides another crucial authentication point, reflecting American import requirements. Its exact positioning, font, and size maintained remarkable consistency within production batches. Modern reproduction dials, while sometimes convincing at first glance, invariably fail under close scrutiny, the printing quality, surface texture, and natural aging patterns of original Stern Frères dials remain difficult to replicate convincingly.

Below 3 examples of the latest known serial sommatore’s to have appeared on the market. The serial range ends as 7,024,6xx.

Also note the differences in patina.

The Movement (Movement Analysis)

At the heart of every Sommatore beats the modified 13ZN-12 calibre, a movement that represents both the zenith of Longines’ chronograph development and a masterclass in purposeful engineering. To fully appreciate this calibre’s significance, one must understand both its origins in the standard 13ZN family and the extensive modifications required to achieve the central minutes function.

The 13ZN-12 features a nickel finished movement with rhodium plating, representing a modified version of Longines’ acclaimed 13ZN calibre. The key modification, protected by Swiss patent 228246 filed in 1942 and titled “watch with chronograph counter hours and minutes,” relocates the minute counter from its traditional subdial position to the centre of the dial, sharing the axis with the chronograph seconds hand.

The addition of the central minutes function prevented the inclusion of flyback capability (also known as Taylor system) due to the mechanical complexity of combining both features within the movement’s architecture. This was accepted in favour of the enhanced legibility that the central display provided.

Examples from order 23086, representing approximately 90% of production, were destined for the American market and can be identified by the “LXW” stamp on the movement bridge and “Swiss” marking below 6 o’clock on the dial, both indicating compliance with US import requirements.

Note 90% of the reference order 23086 were madefor the American market, see the LXW stamp on the movement and ‘swiss’ on the dials below 6.

The base 13ZN movement had already established itself as one of the premier chronograph calibres of the period. Measuring 13 lignes (29.63mm) in diameter, it provided an ideal platform for modification, large enough to accommodate additional complications while maintaining structural rigidity and serviceability. The movement architecture followed classical Swiss chronograph principles: a lateral clutch system, column wheel control, and separate trains for timekeeping and chronograph functions.

The modification to create the 13ZN-12 variant required fundamental reimagining of the minute-counting mechanism. In the standard 13ZN, the minute counter occupied a sub-dial position, driven through a conventional intermediate wheel system. The solution, as detailed in patent 228246, involved creating an entirely new wheel train positioned beneath the dial. This additional mechanism included a friction-coupled wheel that would advance the minute hand incrementally as the chronograph seconds wheel completed its rotations. The friction coupling served multiple purposes: it allowed for independent setting of the minute hand during servicing, provided a safety mechanism against damage if the hand encountered resistance, and maintained consistent torque transmission despite manufacturing tolerances.

From a technical standpoint, the implementation required exceptional precision. The friction interface had to be calibrated to exact specifications, too loose and the minute hand would slip during operation, potentially losing synchronisation with the seconds display. Too tight and the additional drag would affect the chronograph’s amplitude and timekeeping accuracy.

The 13ZN-12 movement exhibits notable differences from the traditional 13ZN calibre (as illustrated in the supply diagram below). Measuring 13 lignes and incorporating 17 jewels, it features shock protection and a balance wheel with glucydur hairspring. The rhodium-plated movement finishing displays the “anglage” technique typical of Longines standards during this era and, uncommonly for traditional 13ZN calibre’s, showcases the refined “Geneva waves” decoration. A distinctive metal ring containing the movement is characteristic of this reference. The “13ZN” inscription appears on the plate beneath the balance wheel, while the “Longines” signature is engraved on both the main bridge and the plate beneath the dial, accompanying the serial number.

It is interesting to note that for collectors, finding or getting these center wheels made in the event of repair is quite tricky. We found a stash of these in the past, which was lucky. In some instances either a new center wheel needs to be found or constructed, and or the axle itself.

Patent 228246

Below the original patent 228246, filed in 1942 with patent number 228246. Movement with chronograph counter, with addition of minute counter, seconds and hour counter on the sub dial. Characterised with the large minutes and seconds hands having a ‘centre rotation’ i.e being in the centre. It also describes where the winding stem is located and the centre wheel that operates the centre minutes and hour counter as highlighted in the second image.

The watch is equipped with hour and minute hands (aiguilles d’heures and aiguilles de minutes), a small seconds hand (petite aiguille de secondes), a minute counter hand (aiguille compteur de minutes), and an hour counter hand (aiguille compteur d’heures). The movement is manually operated and can be started, stopped, and reset to zero using two pushers (poussoirs).

The large seconds hand (grande aiguille de secondes) and the minute counter hand share a common centre of rotation in the middle of the movement, indicating a central chronograph seconds and minutes display. The hour counter hand is positioned on a symmetrical axis passing through the stem (tige de remontoir), suggesting a sub-dial for the hour counter.

Technical details include a wheel driven by the large seconds hand, a friction mechanism to drive the minute counter hand, and components such as a counter-plate (contre-platine) and a bridge (pont) with fixed points for precise operation. An annexed drawing, not included in the image, provides a plan view and cross-sectional view of the invention, further detailing the movement’s structure.

The Case

The case construction of both Sommatore references reveals Longines’ commitment to purposeful design and quality execution. For the 5699, the monobloc waterproof case represented the culmination of Longines’ experience with moisture-resistant chronographs.

Case Dimensions (Reference 5699/23086):

Diameter: 39.5mm (excluding crown)
Height: 10mm (mineral glass excluded)
Lug width: 19.5mm
Crown diameter: 8mm with 24 teeth (typical of Longines crowns from the 40s)
Crown thickness: 1.2mm external / 1.4mm internal
Crown tube diameter: 4mm
Sealed with leather o-ring gaskets

The case features a screw-down case-back with 6 notches (sei tacche) for opening with a special key, and traditional screw-in pump pushers for enhanced water resistance.

The satin finish on the exterior case (running from 6 o’clock to 12 o’clock), while the upper part of the case showcases a circular satin finish (executed on a lathe). The bezel (integrated as a single body with the case rather than a separate component) is polished, matching the upper section of the lugs which also display a polished “thread breaker” on their external rib. The pronounced step between lug and bezel serves as a key indicator when assessing the case’s condition and wear. Original examples display crisp, well-defined edges at this transition. Over-polishing, unfortunately common during routine service, softens these edges and diminishes both aesthetic appeal and value.

The case back exterior bears both the order number and waterproof pairing (as shown below in case back section), while its interior is marked with “Longines” and “Acier Inox.” The waterproof pairing number is then stamped on the inner lug as well. This was done so as to match cases during assembly and service.

Collectors occasionally encounter specimens with mismatched case serial numbers, the result of either hasty service work or end-of-series pieces assembled using remaining factory components.

The original plexiglass deserves special attention, featuring a high, vertical lateral rib. Its wide radius of curvature (characteristic of this period when relatively flat plexiglasses were standard) provides exceptional dial legibility under various lighting conditions.

It should be noted that although collectors naturally look for matching waterproof paring numbers on the outer case back and inner lug, there are many that have mismatched numbers. These waterproof pairing numbers were used to match cases and case backs at assembly and in service and so a mismatched example shouldn’t be dismissed for this reason alone.

The Casebacks

Iconic sei tacche (six notches) screwback case. Above the order number ‘23086’. Below the WP (Waterproof pairing number). The purpose of the WP numbers was so that case-backs and case’s would not get mixed up during both assembly and service.. Even so, there are still some with mismatched WP numbers.

There are at least 3 types of font used on the case backs, all are stamped and not engraved. See examples of 3 different versions above.

There are also a few cases that have popped up whereby there is an extra engraving on the outer case back. 371 and 372E. Most likely bought by the 371 and 372nd Engineer Brigade.

Above and below a very interesting example owned by Lt Colonel TL Schmidt, as sold by Bulang and Sons. Read more about it on https://bulangandsons.eu/blogs/watch- talks/the-dare-devil-inventors-longines-sommatore-13zn-an-original-owner-watch-story/

Exceptionally rare original Longines case back wrench for opening the ‘sei tacche’ (six-notch) screw-back cases of the Sommatore 13ZN-12 reference 5699. Marked ‘LONGINES’ on the handle, this specialised tool features six precisely-positioned teeth that correspond to the distinctive notches on the waterproof case back. These wrenches were essential for properly accessing the movement without damaging the case and are seldom encountered today, making them highly sought-after by collectors of these grail-level chronographs.

Beyond its engineering brilliance, the Sommatore tells the story of an era when manufacturers pursued mechanical excellence without compromise.

Reference 5161 with Order 23297

Reference/Cliche 5161, order 23297, represents the rarer variant of the 13ZN-12 Sommatore that is quite different from the Reference 5699 previously discussed. This is a 39mm snap-back case, with square pushers and distinguished by crab-shaped lugs, an entirely distinct case construction.

Where the 5699 was built as a professional’s tool, the 5161 offered a unique elegance for those seeking an alternative within the Sommatore lineage.

It is considerably rarer than its counterpart under order 23086, with fewer examples appearing in the collector’s market, and even fewer with original dials. Production appears limited, and they were allocated all over the world, including South America, Lebanon, Cuba, Peru, and in Europe, notably Spain, Italy and Switzerland. This global distribution pattern speaks to a different marketing strategy than the America-focused 5699, suggesting Longines saw the 5161 as appealing to a more diverse, international clientele.

The dials of Reference 5161 exhibit notable variation, contingent upon the case material. In contrast to the radium-indexes characteristic of order 23086, these dials typically forgo luminescence, featuring instead telemeter or tachymeter scales, or applied indices in the case of yellow and rose gold iterations. Indeed, Reference 5161 extends beyond stainless steel to include these precious metal variants, enhancing its allure.

Manufactured towards the conclusion of the 13ZN’s production cycle, it coincides with the introduction of the 30CH in 1947, a chronograph developed by Longines as a more cost-effective alternative.

The timing marks the 5161 as a transitional piece, lacking the robust waterproofing of Reference 5699 but offering a refinement that appealed to civilian professionals and customers who valued elegance alongside functionality.

Collectors occasionally encounter specimens with mismatched case serial numbers, the result of either hasty service work or end-of-series pieces assembled using remaining factory components.

Order 23297 is substantially different from 23086 in that the case is not waterproof (not screw-back) but snap back with square pushers with these crab style lugs. Actually, the case construction is completely different. Rarer than the 23086, they are not seen on the market so often, it is estimated that around 400 examples were produced, some to South America, most to Europe. The dials vary between different case metals, but the dials do not have radium indexes like that of the 23086. There are some differences in dials i.e telemeter and tachymeter scales, or applied indexes for the gold cased examples.

Below, see an interesting example intended for the South American market: the presence of three stars on the dial, a subtle yet significant marker prized by collectors. As previously noted, the 13ZN-12 derives its distinction from a modified 13ZN movement, incorporating the centre-minutes function, a feature retained in Reference 5161. This modification precludes the flyback mechanism, yet the design remains exceptional.

Ref 5161, order number 23297. 3 stars on the dial indicates, watch for the South American market.

Below a very interesting 23297 delivered to South America. On the right its period original dial with patina as expected on such a watch. The owner a dear friend, embarked on a great project with Longines whereby they made a new dial for the watch based on the the original. Note the vertical finishing on the dial and great printing on the fonts showcasing what the heritage workshop in St Imier can now achieve. Thanks to Tanguy for allowing me to show this great example in the article.

You can find watches with customized dials of various types: geographical indicators (like ‘Fab. Suisse’, ‘Suiza’, etc.), medical scales, or references to important importers—as seen in models featuring three stars that indicate the famous Peruvian jeweler ‘Casa Welsch’. Also notable are the versions with applied gold indexes, typically paired with gold cases as seen below.

Above, both 5699 and 5161, a side by side case comparison. Note the iconic crowns, both with 24 teeth. Also see the notable differences in the case designs.

Order 23297, in stainless steel “Staybrite” are constructed in three parts with snap-on case back and lever pushers. The case diameter without crown measures 39.10mm with a case height excluding mineral (from the edge of the carrure to the domed part of the case back) of 7.50mm. It should not be forgotten that the lugs, very curved downwards, create additional bulk to the overall presence of the watch which means it wears large and has a nice presence on the wrist. The crown diameter measures 7.3mm again with the usual 24 teeth.

The precise measurements are as follows:

Case diameter without crown: 39.10mm
Height of case excluding plexiglass (from the top of the case to the curved part of the case back): 7.50mm. It should not be forgotten that the lugs, which are very curved downwards, create a greater bulk
Inside lug width: 19.80mm
Crown diameter: 7.15/7.3mm
Crown thickness: 2.0mm (in the upper part where it is tapered)

This combination of substantial case dimensions, particularly for the period, alongside the technically sophisticated 13ZN-12 movement with its centre-minutes function, elevates the Sommatore beyond merely a functional instrument to an object that embodies both technical achievement and aesthetic refinement.

The 5161’s three-piece construction took a different approach, prioritizing elegance over absolute water resistance. The snap-back design allowed for a slimmer profile, while the distinctive crab-claw lugs created a more refined appearance on the wrist. Despite lacking the 5699’s waterproofing, the 5161’s case construction was no less sophisticated, with careful attention paid to the fit between components and the quality of finishing throughout.

Double Scale white

The above watch features an external blue telemetric scale calibrated to 1000 meters (a basic 1-mile version also exists). It includes an internal concentric black sexagesimal scale marked in fifths of a second, along with a concentric inner seconds track. The dial displays Arabic numerals either with radium luminous material or pad-printed as shown in the photo. The dial surface has a semi-gloss, single-tone silver-grey colour with vertical brushing. The hour and minute hands are either thin ‘baton luminous’ style with radium or blued leaf-shaped as depicted in the photo.

Silver Triple Scale

Blue external scale of tachometric type base 1000.
Internal 1km base red telemetric scale.
Internal sexagesimal scale to 5″ of a second.
Black decalque Arabic numerals 1-12.
Semi-gloss silver grey finish with vertical brushing.
Burnished blue “Leaf” type hour and minute hands.

An 18K yellow gold ref 5161, order number 23297

Antiquorum Hong Kong, Lot 108 (2023)

Case & Movement No.: 6’738’368
Year: 1944

The case construction on the yellow gold versions is identical to that of the steel 5161. Above and below, are a few examples that have appeared on the market.

This particular example, invoiced to Girod, the Longines agent in Spain on 22 January 1948, represents one of the final deliveries documented in the Longines archives. Girod & Company, established as Longines’ exclusive distributor for the Iberian Peninsula, had developed a reputation for handling the manufacturer’s most prestigious offerings. The late delivery date places this watch among the very last examples produced before Longines transitioned focus to the caliber 30CH cthat would replace the 13ZN family.

Of all the 5161 variants, the rose gold examples represent the absolute pinnacle of rarity within the Sommatore collecting universe. While reference 5161 itself is considerably scarcer than the waterproof 5699, the rose gold configuration is the rarest Sommatore variant to appear on the market. Fewer than half a dozen confirmed examples have surfaced in documented sales or serious private collections over the past several decades, making finding them genuinely once in a decade events for even the most dedicated collectors.

The 18k rose gold case construction has the same 39.10mm diameter and crab-claw lugs of steel examples, but the precious metal transforms the watch’s entire character. The warm, honey-toned gold creates a contrast with the silvered dial. The characteristic step between bezel and lugs a crucial authentication point on all 5161 references, typically shows sharper definition on rose gold examples due to the metal’s resistance to the corrosion that often soften these transitions on steel cases.

Rose gold examples typically feature applied gold hour markers crafted from matching 18k stock, hand set during dial assembly by Stern Frères’ craftsmen. These three dimensional indices, secured with microscopic pins invisible to casual observation, create depth and richness that printed numerals cannot match. The execution quality of these applied markers provides crucial authentication points, as their precision placement and uniformity of finish reflect manufacturing standards that modern reproductions struggle to replicate convincingly.

The movement finishing in rose gold examples often received enhanced decoration beyond the standard 5161 specification. While mechanically identical to other variants, documented examples frequently display additional perlage on the bridges and more lustrous rhodium plating finishing touches that reflected both the higher selling price and Longines’ understanding that precious metal purchases warranted exceptional attention to detail. This elevated treatment extended to hand selection, with only the finest movement examples deemed suitable for precious metal casing.

Rose gold 5161 were priced at approximately 2.5 times the cost of steel variants, placing it in direct competition with offerings from Patek Philippe and Vacheron Constantin rather than tool watches from other manufacturers. This premium positioning reflected not just material costs but Longines confidence in producing chronographs that could compete aesthetically and mechanically with Geneva’s most prestigious houses.

For collectors, rose gold 5161s occupy a category entirely their own within vintage Longines collecting. The combination of extreme market scarcity, genuine once in a decade auction appearances, and the universal appeal of rose gold execution ensures that these pieces transcend typical vintage watch enthusiasm to become trophies for the most serious collectors.

Authentication requires exceptional care, as the rarity has unfortunately made them targets for sophisticated counterfeiting attempts. The rose gold alloy used by Longines in the 1940s displays a distinctive colour that differs from modern formulations, while hallmarks inside the lugs follow specific patterns that varied by destination market.

The fragility of applied hour markers presents ongoing challenges for collectors. Rose gold’s relative softness means well-preserved examples often retain sharper case details than steel variants, but missing or loose indices significantly impact both authenticity assessment and value. The original red minute hand must retain exceptional paint condition, as restoration attempts on this critical element often prove unconvincing under magnification.

From a collecting perspective, the rose gold 5161 embodies everything that elevated the Sommatore above contemporary chronographs, the innovative central minutes function, robust case architecture, elegant proportions, while adding the unmistakable prestige of precious metal execution.

Collecting Guide and Market Insights

The Longines Sommatore 13ZN-12 remains one of the most historically significant chronographs in the vintage market, representing the pinnacle of Longines’ central-minutes innovation. While market dynamics have evolved in recent years, with references like the Mushroom Pusher and Tre Tacche currently enjoying favour, such waves are natural in the collecting world.

In good condition the Sommatore continues to command significant prices at auction and in private sales.
At the time of writing (June 2025) there has been an adjustment in terms of market pricing that reflects broader market conditions rather than any diminishment in the model’s importance or desirability. Indeed, for collectors who understand the technical achievement and historical significance of the 13ZN-12, current pricing may represent an opportunity.

Condition Assessment and Value Factors

The most critical factor in determining value is dial condition and originality. Original dials, even with honest patina and radium degradation, are vastly preferred over restored examples (as is a standard in the vintage market). The distinctive red central minute hand must retain its original paint, and the blued steel hands should show proper thermal oxidation rather than chemical re-bluing. Water damage, significant radium loss affecting legibility, or amateur restoration attempts dramatically impact both desirability and value.

Case condition ranks as the second most important consideration. For reference 5699, the sharpness of the characteristic step between bezel and lug immediately indicates the level of preservation. The sei tacche caseback should retain clear, sharp notch definition. Over-polishing, unfortunately common during routine service, softens these defining edges and diminishes value substantially. Original pushers should operate with firm, progressive resistance, while the 24-tooth crown should retain clear definition despite honest wear.

Perhaps most crucially, the movement and case must match according to Longines archive records. This verification has become more complex due to recent changes in Longines’ extract policy, affecting how collectors can document their watches. Nonetheless, matching numbers remain essential for serious collectors, and any discrepancies should be fully disclosed and reflected in pricing.

Market Outlook

The community of serious Sommatore collectors remains knowledgeable and discriminating. While current market conditions have created more accessible pricing than the peak years, the fundamental importance of these watches, their technical innovation, build quality, and historical significance, ensures continued collector interest.
Authentication should always involve multiple factors: case construction details, dial printing quality, movement architecture, and overall coherence of components.

Restoration and Maintenance Considerations

The complexity of the Sommatore mechanism demands exceptional skill for proper service and restoration. While experienced watchmakers with knowledge of high-end chronographs can certainly service the 13ZN-12, they must understand its unique architecture, particularly the central minute mechanism. Unlike conventional chronographs where general principles often suffice, the Sommatore requires specific familiarity with its friction-coupled minute counting system.

The central minute mechanism represents the primary service challenge. The friction coupling that allows the minute hand to advance incrementally requires precise adjustment, too loose and the hand may slip, too tight and the additional drag affects chronograph accuracy. Factory specifications called for specific torque values, but achieving proper function often requires experience-based judgment rather than mechanical measurement alone. If anything goes wrong with the system, it’s typically the centre wheel that operates the centre minutes, separated by two pinions as detailed earlier in this article. This critical component’s specifications are shown below:

Parts availability presents ongoing challenges for Sommatore owners. While some components interchange with standard 13ZN calibers, the modified architecture means many parts are unique to the 13ZN-12. The central wheel assembly shown above proves nearly impossible to source as a replacement part. When these components wear beyond serviceability, restoration often requires custom fabrication, a costly and time-consuming process that only the most skilled specialists can undertake successfully.

A crucial tip when inspecting any 13ZN-12 movement: always examine the condition of the wheel teeth using a high-powered loupe. Check carefully for any missing or worn teeth, particularly on the centre wheel and associated pinions. Even minor tooth wear can cause erratic timekeeping or complete failure of the central minutes function. This inspection should be part of any pre-purchase evaluation.

The friction-fit components throughout the movement demand particular attention during service. Unlike jeweled bearings that either function properly or show obvious wear, friction interfaces can degrade subtly over time. The coupling between the chronograph seconds wheel and minute counter wheel may develop inconsistent engagement, causing the minute hand to advance erratically. Diagnosis requires careful observation during extended testing, problems may only manifest after minutes or hours of operation.

Dial restoration remains among the most controversial topics in vintage watch collecting. Serious collectors strongly prefer original dials, even with significant radium degradation, over any form of restoration. The radium index’s, present aesthetic challenges as it degrades, but this patina and aging is considered part of the watch’s authenticity and history. When restoration is undertaken, typically only in cases of severe damage affecting legibility, matching the original Stern Frères quality proves nearly impossible. The vertical brushing pattern, the precise printing registration, and the subtle colour variations of aged lacquer all resist convincing reproduction. Any dial restoration, no matter how skillfully executed, significantly impacts value and desirability.

The red central minute hand presents unique restoration challenges. Many restored hands show telltale signs: uneven colour coverage, incorrect shade, or paint that extends beyond the original boundaries.

Mineral replacement deserves careful consideration. Original plexiglass crystals featured specific profiles that contributed to the watch’s distinctive appearance. Modern replacements often differ subtly in height or curvature, altering both aesthetics and functionality. The high-dome profile of original crystals provided clearance for the tall hand stack; incorrect replacements may result in hands touching the crystal interior, causing drag or damage.

Movement cleaning presents unique considerations due to the additional gear trains. Standard cleaning solutions and ultrasonic frequencies that work well for conventional movements may not adequately clean the friction surfaces in the central minute mechanism. Hand cleaning of these components often yields superior results, though it requires significantly more time and skill. Lubrication points throughout the modified movement differ from standard 13ZN calibers, demanding specific knowledge of oil types and application points.

The philosophical questions surrounding restoration extent apply particularly to Sommatores. Given their rarity and historical importance, arguments exist for both minimal intervention and comprehensive restoration. Museum-quality conservation, preserving current condition while preventing further deterioration, appeals to some collectors. Others prefer full restoration to original functionality, accepting that some originality may be sacrificed for usability. No universal answer exists; each owner must balance personal preferences with preservation of historical artifacts.

Professional restoration costs reflect the complexity involved as will be quite familiar to collectors looking to service vintage chronographs compared to time only watches.

Conclusion

The Longines 13ZN-12 “Sommatore” represents the culmination of 1940s chronograph development, successfully solving the fundamental readability problem through its innovative central minute counter protected by Swiss patent 228246. This exceptional reference represents the culmination of all the finest aspects of Longines’ waterproof chronographs during the Golden Age of watchmaking, ticking all the boxes a collector looks for and housing arguably one of the greatest calibres ever made by any watch manufacturer. In many ways, it is the genesis of independent watchmaking.

Available in two rare variants, the waterproof reference 5699 built for military aviators and the elegant reference 5161 in steel and precious metals, the Sommatore showcases Longines’ ability to adapt revolutionary engineering for different markets without compromise. That these watches were produced during World War II, when resources were scarce, makes their technical achievement even more remarkable.

Beyond its engineering brilliance, the Sommatore tells the story of an era when manufacturers pursued mechanical excellence without compromise. The tactile experience of operating these chronographs connects modern collectors directly to this heritage of uncompromising quality. In an industry sometimes distracted by marketing over substance, the Sommatore endures as a benchmark for what becomes possible when technical innovation serves functional purpose, reminding us that true horological greatness lies in the perfect marriage of engineering excellence and practical utility.

References
[1] Patrick Linder – At the Heart of an Industrial Vocation, Longines watch movements (1832-2009) (p574)
[2]Pierre-Yves Donzé – Longines, From a Family Business to a Global Brand (p139)
[3] John Goldberger – Longines Watches (2012), Damiani Editore, Bologna (pp. 214-226)
[4] Konrad Knirim – Longines: Die berühmten Fliegeruhren / Famous Aviators Watches (2009), Heel Verlag, Königswinter (pp. 112-127)
[5] Swiss Patent Office – Patent No. 228246, “Montre à chronographe-compteur” (1942), Federal Institute of Intellectual Property, Bern
[6] Richard Watkins – The Technical and Social History of Watchmaking Technology (2018), Kingston, Tasmania (pp. 445-449)
[7] Osvaldo Patrizzi – Collecting Modern Wristwatches (2008), Guido Mondani Editore (pp. 267-271)
[8] François Chaille – The Chronograph: Function and Fashion (2004), Flammarion, Paris (pp. 134-138)
[9] Gisbert L. Brunner & Christian Pfeiffer-Belli – Swiss Wristwatches: Chronology of Worldwide Success (2007), Schiffer Publishing (pp. 189-192)
[10] Michael Philip Horlbeck – Longines: From Pocket Watch to Wristwatch (2011), Schiffer Publishing (pp. 156-162)
[11] Marton Radkai – Vintage Wristwatches: The Superguide (2020), ACC Art Books (pp. 412-415)
[12] Longines Archive Department – Production Ledgers 1945-1952, Order Numbers 23086 & 23297, Saint-Imier
[13] Wittnauer Company Archives – Import Records and Catalogues 1945-1952, New York Historical Society
[14] Charles-André Reymondin et al. – The Theory of Horology (2015), Swiss Federation of Technical Schools (pp. 298-302)
[15] George Daniels – Watchmaking (2011 Edition), Philip Wilson Publishers (pp. 345-349)

Period Sources
[16] “New Chronograph Features Central Minute Counter” – American Horologist, Vol. 12, No. 7 (July 1946), pp. 23-26
[17] “Longines-Wittnauer: Innovation in Timing” – Jewelers’ Circular-Keystone, Vol. 117, No. 3 (March 1947), pp. 142-144
[18] Swiss Watch Fair Basel Catalogue (1948), pp. 67-68

Additional Archival Sources
[19] Stern Frères Production Records – Dial Orders for Longines 1945-1950, Musée International d’Horlogerie, La Chaux-de-Fonds
[20] U.S. Army Air Forces Technical Order No. 35-10-3 – “Maintenance of Chronograph Watches” (1945), National Archives and Records Administration

Special Thanks
In writing this article as with the others, I had help from a number of friends.
A special thank you to Philipp Hebeisen from Longines Heritage Department for his assistance in researching for this article and for the archive images.
To Roberto Randazzo for many of the images.
Tanguy for kindly sharing his 23297
Franco for his images of his great 5161.
Alfredo (Doctor Steel) for his 23297
Fabrizio Caso for the images of his 5161 in gold.
Bulang and Sons for the images of their 5699. https://bulangandsons.eu/blogs/watch-talks/the-dare-devil-inventors-longines-sommatore-13zn-an-original-owner-watch-story/
Santi from MimandCrocket
Andrea Foffi for his images of various sommatore.