Zenit and Zorki
The Zenit SLR was created simply by modifying the Zorki to take a mirror box, do away with the rangefinder and, of course, put the lenses in a mount with space for the mirror to flip up.
Other minor changes were made but mostly the Zenit was clearly based on the Zorki.
These two cameras are from the early to mid 50's. The Zenit from 1954 and the Zorki from 1953. The first two numbers of the serial number give you the year made, but don't apply this rule to FED cameras which look like Zorkis. (FED did have a date code but it was only used for a few years and then discontinued.)