Hi there, I am new to medium Format and have just bought myself for almost no money a used Mamiya RB67 ProSD. During shooting the first film I forgot to pull out the blind slide one time but I was still able to release the shutter and mirror and to advance the film afterwards. I thought there is a block which prevents this to happen. The back is 6x4.5 ProSD. Is that a problem of the back or of the body (or is it me )?