There are several versions of the Jupiter-12 in Thread Mount, some have metal guards over the rear group. I am able to use my 1952 J-12 (that has stamped serial numbers showing it is a Zeiss Biogon from 1943) with my Leica M8 and Leica M9, but not with my Canon 7 or Canon P. The Canon 7 and Canon P have light baffles, the large rear element with the metal collar hits into them.
This lens required changing the Shim for proper focus, it was way off. That probably explained the perfect glass and $55 price. Now- it is a fine performer.