Right you are Trond. The lens mount was deliberately change to avoid swapping between the two. And the only difference in the back adapters is the shape of the claw. The Mamiya claw is in one piece, whereas the Polaroid claw is split down the middle. In all other respects they are equal. The adapter can be swapped from one to the other (a matter of some screws) or the claws themselves can be moved from one to the other. I chose Polaroid because that was what I already had (CB103 backs, Graflex 23 adapted to Polaroid 600SE etc).