Kalimar SR300 is a rebadged export version of Zenit-EM for the US market.
Some versions are also named Kalimar EM (#80058602 fotoua.com).
Also avilable in black, and ironically with olympic symbol for the Moscow olympics that USA didn't participate in as a protest!
The camera can also be found marked Cosmorex with a Cosmogon lens.
Cambridge Camera also produced one of these with a Cambron plate.
And there are others brands.
Frame size: 24x36
Lens: Helios-44M 2/58
Shutter: 1/30s-1/500s + B