Hi. Since this is a common problem on the Mamiya 645, I'm here to help you with that particular problem. We've managed to 3d print that part for one client. If you have any Mamiya 645 related problems, our team is happy to do the 3d modeling and 3d printing of that certain part that you need.
Here's our Mamiya 645 mirror stop part. From a picture, to a functional part
Here's our Mamiya 645 mirror stop part. From a picture, to a functional part