Not claiming I know what this is...........During the coloial times, England was in a metal shortage and sent almost no coins to the American colonies. They also did not allow the colonies to mint their own coins,(there were some minor exceptions), so many minted their own coins from Spanish silver and copper. This may be a locally made dime/10cent piece.
Does the "F" and "M" mean anything locally? Maybe it was a "Farmers and Merchants" Co-op?
Very cool find!