I have the RB67 Pro-S Mamiya repair manual in front of me and the only parts difference between the 120 vs 220 backs that are different are part #s aRSH6493 vs bRSH6993 / aRSH6484K3 vs bRSH6984K3 / aRHS6491 vs bRSH6991. Do see where they are in the exloded views? How does this influence the 120 vs 220?