My next project is a delightful bit of horological arcana: a Soviet clone of one of the most accurate wristwatches ever produced.

Before the advent of the atomic clock, watchmakers competed to create the most accurate timepiece in "observatory trials" in Europe. Until 1967, the length of a second was determined by the relationship of the earth to the sun, and so observatories had the finest timekeeping instruments anywhere.

The Zenith 135 is a legendary chronometer, winning five observatory trials in a row from 1950 to 1954. The movement was designed with isochronism in mind, and has an enormous balance (for its size) to achieve the most accurate rate possible. The balance wheel is so large that it required the motion train to be completely redesigned to accommodate its diameter.

After WWII, the Soviet Union dramatically increased its manufacturing capabilities across the board, including watchmaking, but a true observatory chronometer-grade watch was beyond its reach. That is, until the plans for the Zenith 135 "somehow" made their way into the USSR's factories. The result is the Vostok 2809, a near copy of the vaunted Swiss calibre!

Vostok made a few changes, including the addition of a few (mostly decorative) jewels and the elimination of most of the movement's decoration. Still, the massive balance survived the reinterpretation, and the 2809 supposedly achieved impressive timing results. 

Pre-service, the watch isn't currently showing much of anything on the timing machines. I'll be doing a full overhaul (including some needed aesthetic modifications), and will see if I can get this bit of Soviet weirdness to post some good numbers!