This is an hypothesis indeed, although I have no clue that the lens was really
disassembled in the past. The lens works great, except this infinity issue.
The focus ring could have been positioned on a wrong helicoid groove ? Is that
what you think ?
I am not sure if there are many helicoids on these IF Telelenses ... I would
love to find a repair manual, or even a single picture of parts for the 300/2.8
AI/AIS.

I am still pretty sure that there is a way to fine-tune infinity on theses
former pro-lenses, which could be both the origin, and the solution of my
problem, if only I knew how to do that...