Flat Button with Eagle on backmark?

[El Cheapo]

I was wondering if anyone can help me identify this flat button...I found it near some civil war breastworks, but not sure if it is older..At first it was heavily encrusted, and looked like a normal flat button..I was experimenting with electrolysis and as the crud fell off the button the eagle w/shield appeared under the shank. Does anyone have any ideas??

thanks for your help!