These cameras were produced by factory "KMZ" (Krasnogorsk) since 1962 up to 1970.
About 781,678 copies were made
Frame size 24X36mm
Lenses(screw mount M39): Industar-50 (3.5/50mm), Helios-44 (2/58mm)
Lenses for Zenit-3M 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-3: the back cover is openable, the new mechanism of cocking lever with