What makes a certain watch collectible? Is it the first to do something technologically brilliant? Has it been worn by a celebrity and imbued with their style and charm? Did it accompany someone on a mission of great daring? Many of the most sought-after pieces fall under these categories, launching them to the very heights of collectability.

Yet the chronographs produced by Longines that held the 13ZN calibre, arguably, fall under none of the above. What is it, then, that has made these pieces so desirable in the eyes of enthusiasts over the last decade? Speaking to knowledgeable collectors of the watch and the head of the heritage department at Longines, we hope to discover what made this manual-wind chronograph, produced between 1936 and 1951, so appealing.

Even before the 13ZN, Longines had a strong connection to the world of chronographs, from the pocket-watch timers they began making in 1878 to the subject of this article’s predecessor, the 13.33Z. This wrist-worn calibre was designed for military purposes and was often adapted to their specific needs.

However, as the brand’s engineers continually improved on the 13.33Z’s functionality, it was clear that a monopusher, or even an arrangement with a second pusher integrated into the crown, was not fit for the purpose. We are speaking of course of the advent of flyback functionality.

The 13.33Z, like traditional chronographs, featured a toothed reset hammer that was blocked by the column wheel when the chronograph is engaged. The design prevented the chronograph from accidentally being reset. Longines engineers discovered that removing this tooth meant that the reset pusher could be activated while the chronograph was still running, causing the hands to all jump back to zero and instantaneously start running again.

This led Longines to move to a set up with dual pushers, one at two and the other at four o’ clock, an arrangement that was best suited to using a flyback chronograph. This is evidently before Breitling patented such an arrangement of chronograph architecture in 1934.

This flyback function was particularly important for pilots and their navigators, who had to time their movements in the air precisely. Being able to say, with accuracy, that you have travelled west for 10 seconds, north for 30, then east for 15 – all without missing a fraction of a second between the changeover – is crucial to your squad hitting its target.

It is also important to note that this was still the early stages of the industrialisation of the watch trade. As wristwatches were starting to become more practical for working use, demand for them grew, and means of production had to keep up as well. The 13.33Z represents one of the last chronographs made by Longines to come from these older, slower, and smaller production processes, meaning it had degrees of hand finishing, and each movement had individualised aspects that related to its maker.

According to Bernard Portal, the manager of the World Heritage Workshop at Longines, the need for the flyback function was clear to the brand before they introduced the 13ZN. “We were already using the flyback on the 13.33Z around 1927,” he says. “At the time we did not consider it necessary to file a patent; perhaps this was an oversight. The flyback function that we initiated on the 13.33Z was, from what we know, a request from Italian pilots.”

This was not the final step Longines took before releasing the 13ZN: they would file a patent in 1935 for the flyback chronograph calibre 15”’. What confuses fans of the era, however, is why Longines chose to patent this movement, given that it was based on the Valjoux 22GH. This large movement, 34mm in diameter, wouldn’t see high levels of production. According to George Pakkos, a knowledgeable long-time collector of vintage Longines, “very few were produced [and fewer] than 10 have been seen with flyback”.

The 13ZN, in comparison, was smaller – 29.8mm in diameter, featuring 17 jewels and a straight-line lever escapement arrangement. The balance was bi-metallic, and the calibre featured a Breguet balance spring, offering a degree of defence against magnetism and temperature changes. The calibre was decorated with nickel or gilt finishing, depending on the material of the case. While Longines didn’t choose to file a patent for this movement, they did use it to popularise and commercialise the flyback function in wristwatches.

Portal believes the primary business reason for the new calibre was to take advantage of the rapid advances in industrial machining. “The essential advantage of 13ZN compared to 13.33Z is, mainly, the manufacturing cost,” he says. “It is estimated that the manufacture of 13ZN amounted to several tens of percentage savings compared to [that of the] 13.33Z. We started with a new movement base [as] industrialisation was starting to take hold, [and] the simplification of the assembly process [made] it possible to speed up the assembly of movements. Our technical office, [which is] what we would call engineers today, constantly innovated throughout the golden age of our manufacture. They worked on simplifying the construction of components, simplifying the stopwatch minute-jump functions, for example.”

Pakkos agrees that the economies of scale of the calibre 13ZN were in contrast to the 19th-century manufacturing techniques used for the 13.33Z. “The 13.33Z was very expensive for Longines to produce, [with its very] high-quality finishing,” he says. While many components of the 13.33Z had to be finished by hand, this was not the case with 13ZN.

Despite the 13ZN being known for its flyback functionality today, there are early transitional models that didn't have this function. “The 13.33Z and the 13ZN were compatible for the cases and dials [that Longines were producing], and since the majority of the 13.33Z cases were mono-pusher cases, we used them,” says Portal. “So, at the very beginning of the 13ZN, we had a few single-pusher non-flyback models. Then, and very quickly, we only made twin-pusher cases. For these models, it is very rare not to have a flyback.”

There were also several twin-pusher calibre 13ZNs that also did not feature flyback functionality. “That some 13ZN models are not flyback is, I think, [the result of] a request from some of our customers,” says Portal.

The 13ZN also came in the kind of dizzying array of dial configurations that were typical of watch manufacturing of the era. While the basic formula – twin pushers and a two-register dial – remain consistent, the calibre 13ZN was fitted to a variety of cases. Dials could feature tachymetre and telemetre scales, and sometimes both. Others, such as the reference 3879, featured a pulsation scale while a few, such as reference 3756 – aimed at the casual wearer – did not feature any scales at all. Cases could range from 34mm to 38mm in diameter and could feature straight, articulated or hooded lugs, the latter being seen on reference 4442.

Pakkos says that while references powered by the calibre 13ZN have generally proven to be quite collectible, some are more sought-after than others. “The most desirable references are, without a doubt, the waterproof chronographs … most notably the ‘mushroom pusher’ reference 4270, the tre tacche 4974, the sommatore 5699, and also the reference 5415 with the double lip,” says Pakkos.

The reference 4270, produced for just five years between 1937 and 1942, featured another Longines innovation. The company filed a patent in 1938 for the mushroom pushers that made the case waterproof, making use of a proprietary design coupled with materials such as cork within the pushers to create a “waterproofing sleeve” to ensure chronograph functions could be operated without any ingress of moisture. The technology was paired to a substantial 37.5mm stainless-steel case with two-part construction and a stepped bezel. Italian enthusiasts nicknamed the reference the pulsanti ad ombrello.

While most examples of the reference 4270 were cased in stainless steel, a very small number were in solid 18k gold. These watches featured round pushers, an easier design to forge out of a precious metal, and had cases that were underpinned by the same proprietary waterproofing technology.

Another notable waterproof reference was the 4974 – also called tre tacche by Italians, referring to the stepped case – featuring the same pushers as the gold reference 4270.

The only notable variation on the calibre itself was the 13ZN-12. First produced in 1942, the difference was that it incorporated another level of complication – the ability for the chronograph to record 12 elapsed hours displayed in a sub-dial at 3 o’clock. There were four central hands: the standard hour and minute hands, a central blued hand showing elapsed seconds, and a red hand recording elapsed minutes. This added complexity makes it, in Portal’s words, “a gas plant to service”, but also makes it one of the most desirable examples of the 13ZN. Another reason for its popularity is the very modern proportions of the reference 5699 Sommatore case that the 13ZN-12 was paired with. At 39.5mm across, it was oversized for its era but well suited to modern tastes.

While it is undeniable that the last decade saw renewed interest in the 13ZN, reflected in the auction prices fetched by certain references, Pakkos thinks the calibre has always had a loyal following. “These watches have long been collected by astute collectors,” he says. “If there was an increase in popularity I guess [it was because] social media had a large impact, as did books published on Longines within the last decade, most notably by John Goldberger.”

To Pakkos, the 13ZN represents something of a rare gem even from an era he calls the golden age of watchmaking. “I consider it one of the most beautiful chronograph movements ever produced,” he says. “[It is] not perfect, but you must consider [that it was made] in 1936; the design at that moment was unique, made all in-house by Longines. It was not a Valjoux- or Lemania-based movement [at a time when] even Patek was not producing their own in-house calibre for their chronographs, rather finishing them to a very high standard. [The 13ZN wore] high-quality finishing on the bridges and excellent anglage on the levers.” In terms of technology, it was cutting edge – it featured a Glucydur alloy balance wheel and a Breguet oversize balance spring, Pakkos notes.

The variety of movement finishing and complications – flyback, non-flyback, central minute counter – meant the 13ZN presented a wide variety of niches for collectors to drill down into. “This, along with the larger size, fantastic cases, and dial designs, meant that the chronographs with calibre 13ZN are some of the most collectable and interesting chronographs from the period,” says Pakkos.

Independent watchmaker Torsti Laine, who in 2020 had the opportunity to work on the movement, says, “[It is a] beautiful design; I always liked the fact that the seconds wheel had a bridge on top instead of the wheel on top, as in a classic Valjoux movement. It’s a respected, difficult-to-get and expensive movement nowadays.”

Laine created a one-off watch, the Laine Watches AKIL 13ZN Piece Unique, on request from a customer. The watch’s aesthetic pays homage to chronographs from the first half of the 20th century, and the customer specially requested that it be powered by a vintage 13ZN, testament to the calibre’s popularity. “While of course modern chronograph movements have several improvements, in my mind, aesthetically, a lateral clutch layout is hard to beat,” says Laine.

By 1951, the 13ZN had become a relic of the past. Just like the 13ZN had represented efficiencies in manufacturing over the calibre 13.33Z, further advances in industrialisation meant that a new movement design, in the form of the calibre 30CH, was replacing it. “It was the complexity of the assembly that ended the life of the 13ZN,” says Portal. “The 30CH benefited from galloping industrialisation which has made it possible to assemble this movement much more easily and at a lower cost.”

While the 13ZN required less manual intervention than the 13.33Z, a significant number of components still had to be finished by hand. “All the functions of the chronograph – the various cams, hammers, and the like – required manual finishing,” says Portal. “Each watchmaker had their own finish, and it was impossible to exchange the supplies of two movements assembled by two different watchmakers. This was no longer the case with the 30CH.”

Increased mechanisation also had its downsides. Just like the move from the calibre 13.33Z to the 13ZN had led to a simplification of the minute-jump function, the 30CH meant a gradual phasing out of the flyback complication. “Strangely, for the 30CH the proportion of non-flyback is very important and even disproportionate. Moreover, when the 30CH was discontinued, with the exception of a single model, Longines ceased to produce any flyback chronographs,” says Portal.

Longines stopped producing chronograph calibres in-house afterwards, resorting instead to sourcing Lemania, Valjoux, and Venus movements. If the 30CH marked the end of an era, we would argue that the production period of the 13ZN marked a time when the brand truly hit its stride. Balancing in-house production and technological complexity with increased production meant Longines wasn’t just able to put out the 13ZN in significant numbers – they benefited from modern movement innovations that meant they were robust. Advancements in waterproofing made the references that featured it understandably attractive.

Technological aspects aside, the calibre 13ZN was paired with cases that are well proportioned even by modern standards and have a wide array of dial styles made by the likes of Stern Frères. While certain references have performed better at auction than others, 13ZN-powered references have long been coveted by seasoned collectors who appreciate what the calibre represented in the history of Longines and watchmaking at large.