That is why it is called RB67.
Pliftz: You didn't say if your camera is a Pro or Pro S model, (or Pro SD) but if yours is just the Pro, all the other model backs will fit it but the others have interlocks. They will not work on the Pro, but if you have a Pro S or SD, you should try to get the proper backs. You will pay more for the Pro S back, but since the interlocks won't work with it, you would just be wasting money. The interlocks prevent certain things like not tripping the shutter if the film is not wound, and the back not straight or the dark slide being in place and probably a few I forgot. The reason the RB67 is called that is because of its 'revolving back' and the fact that it will take 6X7 pictures. You can probably sell the 645 and pick up a 6X7. You can also put a polaroid back on that camera, but I suppose you know that. Have fun. Ric.