Beside the proper lens distance registration is another issue:
The collapsible Fed 3,5/50 lens made for the Fed-1 will not correctly fit the Fed-2. There is a problem locking the lens at infinity, the infinity lock touches the body around the lens mount. For the same reason it can't be mounted on the Zorki 4. Maybe some early Fed 2's will fit the Fed-1 lens, but my Fed lens won't fit the Fed 2.
I have a post war model Fed now from '51 and its uncoated lens will not fit my other Fed 1 made in '54. The infinity lever stops at another point, when mounting the lens the infinity lock stops between the rangefinder windows making it impossible to use. The new Fed lenses however can be used with the infinity lock down on the early models.
Early 'uncoated' Fed-1 lens: will not fit the late Fed and other models.
Later 'coated' Fed-1 lens: will fit the Fed-1 and Zorki-1 models, but not on the Fed-2 or Zorki 3/4/5/6 models.