Your friendly neighbourhood nut and bolt shop should be able to do that in a trice.
Where are you?
When I go into my local shop with a bolt or similar to identify, they roll it around in their fingers, peer at me over their glasses, and say "is it 3/8 UNC?" or some other bit of coded language. They then shuffle off out the back and come back with the precise thing.
There used to be a shop in Brighton Road, Surbiton. The woman in there was magic!
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