US button help plz

[PennDirtDigger]

Found this around Pittsburgh. I haven't seen a back like this before....

 

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[GaRebel1861]

I’ve never seen that back type either They are normally two pieces. I assume that you already know that the pin goes on a enlisted Army soldiers dress uniform collar.

 

[Tack Driver]

Maybe a horse bridle

 

[Professor of Engineering]

Nice find!

 

[nhbenz]

This type of back (Type V) was introduced during WWII. The ones made at that point had a steel insert holding the pins, where as that piece on ones made post war (and up until 1970) was made of brass. I think that piece on yours is brass, else it would have rusted off.
Dating Metallic Insignia: Collar Disks