Mamiya RB Back Shell Seals
This USCamera custom seal kit will service one RB 67, RB 67 Pro, Pro S 120, 220, and 645 film back shell. This 1/2 of what I recommend you replace. The other 1/2 is the insert kit. The RB film back consists of the shell and insert. In checking film backs for years, if the shell seals are bad so are the insert seals. I recommend replacing them as a set. You may purchase the complete film back kit here. This is the shell kit only and includes 2 door channel seals on the back cover, 1 back cover hinge seal and 1 back cover door latch seal.
USCamera RB 67 film back placement guide will help with installing the shell kits. Look here