It is well documented and well known that prewar Fed 1 cameras had a 39mm thread that differed slightly from the Leitz standard M39 thread preventing lens interchangeability; and that all postwar Feds and Zorkis do conform to the Leitz standard.
It is also well known that lenses with a tounge cam (Leitz, Serenar 135mm for example) cannot be used on Fed/Zorki because the cam follower sled on FSU cameras (as opposed to the cam-following roller on Leicas) should not be asked to ride up the steep shoulders of the tounge cam, and the thread on the FSU camera's lensmount may not start at the Leitz standard position, so the cam might not screw in to end up at the necessary 12 o'clock position.
What is undocumented as far as I know is that Fed 2 won't accept Leitz lenses...the cam rings on my '38 90 Elmar and '54 50 Summicron are so much thinner than the ones on the FSU lenses that the Leitz lenses won't mount properly on the Fed, because the cam-following sled on the Fed misses the cam ring on the Leitz lenses completely. Too much space between the lens flange and the cam follower sled of the Fed 2 lets the thin cam ring of the Leitz lenses fall into the space between the lensmount and the cam follower sled.
It's not the thread...the Industar-26 and Jupiter-8 screw into
the Leica...but the position of the cam follower in the Fed body.
This problem doesn't arise with the thicker cam rings of the FSU
lenses. The FSU lenses can be mounted on both Leica and Fed 2.
