the mamiya press film holders can be attached indeed via the adapters. they are famous for keeping the film absolutely flat. I've got a k holder + both masks (6x6 and 6x4.5). the k holder is easy to find but the masks are usually missing. for the press models there are even finder masks as well. quite handy, but in the end it's a pain to have to look through the little windows at the back for the next number advancing the film cautiously via the knob.

there is no better thing than a lever. there are graflock lever backs for 6x6 (mine is labeled "singer"). they fit onto one of my mamiya 23's with the rare graflock back. they should also fit onto your rb.