Most everything I've read says the backs are interchangeable. I was just about to pull the trigger on an SD back for my PRO then I encountered the last post on this thread which says it didn't even fit his PRO-S:
http://photo.net/medium-format-photography-forum/00UaZz
"I recently purchased a Mamiya RB67 Pro S with the Pro S revolving back. I later purchased a Pro SD 120 film back to go with it because it was mentioned everywhere that they are interchangeable.Come to find out my Pro SD film back will not fit onto my Pro S body. I assume you need the Pro SD revolving back as well to attach the Pro SD film back to it."
So what am I to think now? Was that poster just drunk, confused or mistaken?
http://photo.net/medium-format-photography-forum/00UaZz
"I recently purchased a Mamiya RB67 Pro S with the Pro S revolving back. I later purchased a Pro SD 120 film back to go with it because it was mentioned everywhere that they are interchangeable.Come to find out my Pro SD film back will not fit onto my Pro S body. I assume you need the Pro SD revolving back as well to attach the Pro SD film back to it."
So what am I to think now? Was that poster just drunk, confused or mistaken?