M39 SLRs use the same 39mm screw mount as early Leica cameras and their various copies. The only difference is the mirror in between, which makes rangefinder lenses unusable on SLRs and vice versa. This mount was only used in the Soviet Union for a few early Zenit cameras before they switched to M42 screw mount.

M39 SLR lenses will not focus properly on LTM rangefinder cameras.