The infinity focus is the issue with why you would need one vs the other:


RB67 lenses should work with both the "RB67" and "RZ67" adapters and have infinity focus with both.

However, RZ67 lenses shouldn't have infinity focus with the RB67 adapter, 
although they should fit and function for closer work.
Of course, RZ67 lenses should have infinity focus with the RZ67 adapter.