I purchased my Mamiya 6 brand new when they first came out (several years ago) and there is even a warning in the instruction book regarding the installation of the 50mm.
It has to be put on very straight and carefully. This is different that the 75 or 150, both of which can be rapidly inserted and twisted on. Take your time with the 50 however! This lens sticks further back into the camera body than it's two brothers.........................................p iniongear