I just got a used J-12. It does have a big rear end. It fits my CLE, my Hexar RF, but not the Bessa R2. On the R2, it's jammed against the rear of the compartment (seems unsafe) and won't turn properly to the 15 M and Infinity focus positions. I'm not brave enough to see if it actually puts pressure on the shutter or is just very, very close.