This early Zenit Prototype was made by the KMZ factory near Moscow, USSR in 1950.
Essentially, a Zorki 1 with mirror and prism system (SLR) instead of rangefinder. The film loads from the bottom, as in screwmount Leicas/FED/Zorki.
Most of its characteristics are similar to the series model but the prism housing and the eyepiece are one of a kind.
Dr. Milos Mladek, well-known expert in Russian cameras writes in his expertise:
After a thorough examination of the camera and after having compared it extensively with both a Zenit prototype and a “Square Logo” model Zenit, we consider this camera a KMZ factory research model that was very probably made just before the series production of the regular Zenit to test the new rounded shape of the prism housing and moreover to test a special diopter-adjustable eyepiece (+/- 3 diopters, three lenses in a non-rotating mount, very well made - regrettably it later did not make it into series production).
It always was highly unlikely that KMZ would have started series production of the regular Zenit body without having tested the new shape of the prism housing: here is the test sample. - It comes with a later lens, cap and case
Photo credit: © WestLicht Photographica Auction