Originally posted by Kerrowdown Lots of help already given, so I'll just say welcome.
Thank you all so much! Very helpful and quick responses.
To answer the questions:
Na Horuk: It is definitely something with the lens. Other lenses screw on to my camera just fine, and the lens in question has the same problem when I screw it onto my Mamiya 500 DTL (I have two of them, same problem on all cameras I try this lens on.)
Pacerr: Thanks! I tried electrical tape (1 layer thickness wasn't enough, but 2 layers worked!) I put 4 groups of two, one group at each side (North, East, South, West if you're looking down at the rear of the lens) and that seems to keep the lens in place.
Stevebrot: The manual/auto switch will only work when the lens is on the camera, so I can't look at the rear of the lens and see whether the pin retracts or not while moving the lever from "A" to "M" but I tired taking the lens off both in M and A mode, and the pin sticks out in both. But if I push my finger in the pin it pushes in quite easily.
Reh321, as Stevebrot mentioned, there is no adapter, the lens screws right into the camera.
Thanks everyone for the timely and thoughtful replies, I believe this problem is solved!