there was a gentleman in Hamilton who used to repair cemented ‘doublets’ that had (or were starting to) delaminate .... I believe I originally found him on an astronomy site. Forgot his name. but someone like that might be able to work on the prism. As for rangefinders ... probably if you can find someone who can service older Leica ‘M’ film cameras they may be able to take a stab at your rfdr. There is a semi silvered coating on the beamsplitter on Leica rangefinders and perhaps similar construction on your B&S but IDK .... if so a ‘pro’ who works on old Leicas might be the best option. Incidentally, not that you asked, but personally I would stay clear of any British repair service for optics - they are ALL shoddy.
