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ZENITH Chronometer Prototype, Cal. 135

CHF 3,500


Extremely rare and well-preserved fine ZENITH observatory chronometer prototype Cal. 135 created in testing department early 1950s. As almost any other prototype it has no serial number, so it never used for actual competitions, however Zenith heritage department has confirmed by phone, that they were testing such Cal. 135 chronometer with Glucydur balance at 18.000 alt/hour to try to achieve better results. Very unusual configuration with modified regulator and oversized Glucydur balance makes this timepiece virtually unique and very important.

Case: three body, gold plated, circular case, polished bezel, with glazed transparent display back, large crown. Dial: silvered matte dial with outer minute track, Arabic numerals, blued dauphine hour/minute hands, and baton seconds hand, engine-turned subsidiary seconds. Movement: gold plated Cal. ZENITH 135, frequency 18’000 bpm, oversized Glucydur balance. Movement and dial signed, Diam. 36 mm; 

Ephrem Jobin was the brains and hands behind the Zenith Calibre 135. Introduced in 1948 at the height of a battle to achieve increased precision fought among several watch manufacturers in the Observatory competitions, Caliber 135 included several innovative technical features that enabled it to win numerous prestigious awards and prizes, including an unprecedented 5 consecutive Neuchâtel Observatory chronometry prizes from 1950 to 1954. In all, the movement garnered about 200 honors, two-thirds of which were first prizes.

Competition chronometer-quality wristwatch movements are not “ordinary” C.O.S.C. certified movements. These movements were designed and finished to excel in the rigorous world of Observatory Competitions, as conducted in Geneve and Neuchatel in the years 1944 through 1967.

Watch is in overall very good condition 8/10.


Brand Zenith
Model Observatory Competition Chronometer, Prototype
Year Early 1950s
Movement Hand-winding
Calibre Zenith Cal. 135
Case material Brass
Case size Diameter is 36 mm
Bracelet N/A
Features Observatory Chronometer, Rare prototype Cal. 135
Box/Papers No box, no papers


In fully working order. Very good condition 8/10, minor surface scratches, scuffs and discolorations through the age. Off-centred disc regulator and adjustment functionality was originally modified by Zenith adjusters, who tested the movement for competitions. See photos.

Watches are sold as-is, and there is no guarantee of to be waterproof or movement accuracy, due to the nature of vintage watches. Please view all the pictures carefully as they form an integral part of the description. Any defects not mentioned but visible in the pictures will be considered as described.


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