Zenith caliber 135-O

Developed from 1945 onwards by Ephrem Jobin, the Calibre 135 was produced from 1949 until 1962 in two distinct versions: a commercial variant, and a second iteration made solely for taking part in chronometry competitions at the Observatories of Neuchâtel, Geneva, Kew Teddington, and Besançon. These "O" movements, which were never commercialized nor cased in wrist or pocket watches, underwent exhaustive testing through drastically different temperatures, shocks and running in 6 different positions, consistently delivered optimal chronometric performance with minimal variations in rate. With over 230 chronometry prizes, the Calibre 135-O holds the most awards of any observatory chronometer calibre in the history of watchmaking