It's for sure a pewter button, but I can't nail down the design. For some reason it reminds me of a CW Marine button. Looks like the high point (diagonal line) that is showing may be the remains of an anchor, and I can definitely see an eagle's leg & foot and maybe one wing. It's the design around the edge that is throwing me off. It's just a long shot, but maybe look at different eagle & anchor buttons and designs from the Rev War through Civil War (Marine, Navy, etc.). You may come up with something. Hope this helps. Nice find by the way!