In article , jrml@dml.ac.uk says...

>Can anyone tell me the purpose of the small metal
>structure that sits (or used to sit) on the aperture ring of
>Nikon lenses.

Ah, you mean the infamous non-AI (or pre-AI) meter coupling
clip that sits on the aperture ring at f5.6 and engages a
pin on non-AI (Automatic Index) bodies, huh? Oh, that's
what it is ;-) (Actually, it is a nice non-slip thumb stop,
useful when picking up or putting down lenses face down on
a shelf.)
Hope This Helps