@ shutterfinger : Thank you for the suggestion; indeed thread locker (loctite 222) is what I'll use if it turns out I have to perform service myself. Trouble is, the face ring will not unscrew by more than 1/2 turn. I don't have the same approach on a new lens under warranty (at least in principle) as with a garage sale find.
I know the definition of entrance pupil, f-number, etc. I contributed to design, align, test, several astronomical instruments in the mm, submm, and NIR ranges.