Does anyone know what this is?
 

Brass with the letter G stamped in the middle.

a G

Looks like a key for winding an old mantle clock
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A key for a medieval G-String?
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this

a skeleton key for a chastity belt???

Bunch of animals. Lol
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Damper key for fireplace or stove

A key for a Security guards's clock. as they make their rounds they are at different locations along the route. they insert and turn, it imprints a letter that coresponds to that location on a Time wheel. that way the company knows you did your route and at what time you were at each location.

That's the best answer by far!

That was my initial thought, but the key shown is too simple to differentiate locations in watchman's rounds. That looks like the pin that holds up the tailgate to my 4x8 trailer.

Is that a set screw on the end ?
Not a key then , probably attaches to something . Maybe an adjusting knob ?
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Its a henweigh

paper weight

What's a henweigh :uhuh:

That is the door key for a Garrison wood stove.

And we have a winner! I did some searching around and found a pic with the key in use, here;
http://albany.listlux.com/a,53,20713...uilderland.htm

No searching involved. Wood stove is right next to me as i type. Though my key is steel, I remember someone having one with the brass key.

Way to go Pete you should be a sleuth