Looking at the product specs at Mamiya's web site, it seems that the the backs and inserts for the RB and RZ cameras are different and not interchangeable. The inserts all seem to be matched to the backs as well. Only 220 inserts with 220 backs and 120 inserts with 120 backs match up. The 645AFD uses a single magazine and insert for both 120 and 220. The 645AFD insert is adjustable by rotating the pressure plate. I don't know about the earlier models.
As posted earlier, all manual focus Mamiya 645s switch between 120 and 220 by switching inserts. In both the Mamiya M645 Super and the Mamiya M645 Pro the insert itself tells the camera (and film counter) which length film is installed. As far as I am aware, it works the same in all the other models.
As far as the C series Mamiya TLRs are concerned, it seems to vary by model. On my C330, I switch by rotating the pressure plate, and the film counter is automatically reset. On my C220, I switch by rotating the pressure plate and manually resetting the film counter.
If anyone would like to give me a Mamiya 6 or Mamiya 7, I'll report back
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Matt