Not really testing anyone this time, more just want to know what it's for / does / anything missing?
Appears to be an aluminium pop rivet, no evidence that somethings been ripped off or was held on by it.
Difficult to tell but at the minute I'm going to err on the side of saying it's a blind hole that it occupies.
The second image isn't ideal at showing where it is, but it's in that area.
Ignore that piece top, left in the first image, it's part of my enormous engine stand.
(https://lirp.cdn-website.com/521a0932/dms3rep/multi/opt/uk+rivet-e00b3fdf-1920w.jpg)
(https://lirp.cdn-website.com/521a0932/dms3rep/multi/opt/other-ac1795b2-1920w.jpg)
I won't be back at my workshop for a couple weeks, but remind me and I'll have a look on my engines.