These cameras were produced by factory "KMZ" (Krasnogorsk) since 1968 up to 1973 and since 1977 up to 1978.
About 889,758 copies were made
Frame size 24X36mm
Lenses: Helios-44 (2/58mm), Industar-50-2 (3.5/50mm),
Lenses for Zenit-B are here
Adjustable shutter speeds - 1/30, 1/60, 1/125, 1/250, 1/500 of a second and "B".
The main difference from Zenit-3M : mechanism of mirror and lever cock were changed.
This camera is like camera Zenit-E but It hasn't selenium light meter