My old Pentax lenses have a small white dome between the aperture ring and the focus ring. See the photo below:
The clear white dome calls to mind the larger white plastic domes used in light meters, but there's no such thing in this lens (and that'd be a very strange place for one). You can't press it or anything; it's not a button or control. It's just a white plastic bead half-embedded in the metal. It's common to many or most Pentax manual focus lenses (take a look at any of the K or M primes shown here); I haven't seen it elsewhere.
This dome is on a part of the lens body which does not move. It lines up with f/1.7 when the aperture is set to f/16, or f/2 (unmarked) when the aperture is set to f/22. It's not in a place where it seems likely to provide any meaningful tactile feedback — in fact, when holding the camera in use, I'd characterize it as "out of the way". And it's offset from the orange dot used for mount alignment, so it's not that either.
So, what is it there for? It can't be just to look pretty, can it? It's not very pretty.