Found at 1850’s fort. What is it though?

[history preserved]

Any ideas?

 

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[mojjax]

I think it's a vent to a mattress

 

[SkaBa]

mojjax is correct.

 

[Bruce R]

Or somebody’s boot

 

[TheCannonballGuy]

Because you say it was dug at a fort site, I'll mention that some civil war (and later) kepi-hats and other military hats (and civilian hats) had a similar circular multi-perforated brass ventilator. That being said, it is not the civil war kepi-type ventilator, nor the Model-1881 US Army Dress Helmet's ventilator.