Having got round to taking a ruler to it, the brass nut measures about 3mm across. I suspect I would destroy it if I tried to unscrew it.

I haven’t had much joy identifying postwar crashes in that area. I suppose another possibility is it is a fragment of the RAF S&R pinnace that capsized with three fatalities outside the harbour in 1969. However, I would have assumed that it would have been recovered intact at the time.