Solved!

1976 350SL, was getting slow to go into reverse and, starting recently, would only go into reverse when cold.

Per discussion, I looked for the reverse band adjustment bolt.

It was not a 10mm bolt with 22mm locknut, but a single 19mm bolt. As it had no locknut I tightened it fully and sure enough reverse works fine now.

It was hard to get at so try to jack car as high as you can, and remove the solenoid switch for access.

phew!