The spacer (I think!) is a square plate that fits over the two prongs on the grip that fit into the two holes on the bottom of your RB next to the tripod socket. I think the holes in the base of the mamiya TLRs are wider, so the grip (which also fits the TLRs) will go into a snug fit into the base. On the RB, the holes are narrower, so the plate fits over the two prongs and leaves two smaller spring-loaded prongs which fit into the base of the RB - sorry this is a bit difficult to explain. The upshot is that if you have a multi-angle grip, the spacer should give a better fit to the RB. However, I have a multi-angle grip and no spacer, and it's fine on my RB. If you really feel you need one, all you need is a thin piece of aluminium plate and drill two holes and there you are, however I've never felt the need for this.
