The little arrow above the 5T looks like an "HMG property" symbol - I have the same on the back of my wristwatch
Are there any connections or markings on the reverse? What's the diameter, roughly.
Whatever it is, the thing that I find most curious is the port/stbd asymmetry.
Incidentally, I'd be willing to bet that if you put a Geiger Counter to it, you may find it's fairly radioactive. Not because it's from a reactor, but because it looks like it may have used quite a lot of tritium or uranium based "glow in the dark" paint which was quite common for some years.
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