I just got into good RB67 set, including S and SD backs. I knew the SD back was missing a "silver post" of some sort on the upper left of the back of the camera. Eventually I discovered it is part, or so I believe, of the double exposure system. Without it, there is nothing except good habits to prevent multiple exposures. That is a minor problem as long as I can teach this old dog to always advance the shutter first.
But just to keep the camera as complete as possible I would like to replace it. It looks like it just screws into the body, I don't think it has any mechinical action, It's just a post to prevent something else from moving. That being the case, the solution would be locating another post. Is there any other source short of buying an RB body for parts?