I have an early model with the factory Ai upgrade ring. The knurl on the lens is worn smooth and the
lens mount is so brassed and worn, the lens jumps/shifts when an exposure is made. Looking at this beat to crap, worn lens, it' a wonder that the glass is near perfect.
Unable to find another mount, I used a very sharp and hardened mini wire cutter. I placed two horizontal "chomps" on each of the 3 lens flanges, to allow a tighter grip.
The lens now stays tight and takes extremely good shots.