Identify this piece of hardware
 

David Nebenzahl posted for all of us...


See attached pic (small GIF).

I've found two of these the last few days, one small (~1"), the other
several times larger. Just wondering what the proper name for them is.

(Please disregard mis-drawn left-hand threads.)

Nebby, your pix didn't come through; as it shouldn't...

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Tekkie Don't bother to thank me, I do this as a public service.