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Zenith introduced Caliber 135 in 1948 for the purpose of entering it into observatory competitions that were the rage in the watch industry at the time. The Caliber won numerous prestigious awards and prizes, including an unprecedented five consecutive Neuchâtel Observatory chronometry prizes from 1950 to 1954. In all, the movement earned 200 honors, two-thirds of which were first prizes. To this day, Caliber 135 remains a legend in Swiss watchmaking.
The majority of the Zenith Caliber 135 timepieces were produced in steel, consistent with its intent to be a tool watch. There are some examples in 14k gold, but very few examples out of the 11,000 were made in 18k gold and even fewer have survived in this incredible condition.
The watch features an original and untouched white dial with Arabic numerals at 3, 6, 9 and 12. Whereas the text on the dial is printed, the Zenith star logo is applied in gold.
The design innovations of Cal 135 included a large barrel for improved isochronism and power reserve, as well as an oversized balance which, as part of the regulating organ, plays a crucial role in enhancing precision. In this model, the larger balance generates a higher rotational inertia that allows more consistent performance, without increasing weight or beat rate. Zenith also offset the minute wheel from the central axis so as to provide space for the enlarged balance. The observatory competition version of Caliber 135.0 was equipped with a Breguet overcoil balance-spring, as well as a single or double arrow-shaped index or regulator ensuring balanced friction and enabling optimal adjustment.
The size of the movement was pushed to 30mm (13”) which is the maximum size a watch can be for the Neuchâtel Observatory competition before getting placed into the pocket watch category. The Cal 135 beats at 18,000 vph, features 19 jewels and has a slim movement, at 5mm.
Owing to the exacting nature of producing a chronometer-grade watch, only 11,000 examples of Cal 135 were produced from 1949-1962. There were three variations of the movement, with competition-grade series 1 and 2 being the most desirable, because the finishing of the decoration was the most refined. The only difference in series 1 and 2 is that in the second series the micrometer adjustment was changed from a round-headed pin to a screw. According to our research, the present example, bearing movement number 4663543 is from the second series produced between 1954 and 1960.
Dial: Original, White.
Case: 18k yellow gold, snap back, serial number: 765493.
Dimension: 36 mm diameter, 10 mm thick.
Movement: Zenith Chronometer Grade Cal 135.
Frequency: 18,000 vph
Signatures; Dial, Case, Movement, Buckle.