It is a Civilian-use (not Military) button from the latter 1700s to the early 1800s. Your button is known as a 1-piece solid-cast "white tombac" button. White Tombac is an alloy three metals ...copper, zinc, and arsenic. Apparently, the presence of the arsenic in the alloy prevents formation of the typical oxidation-layer seen on excavated brass-alloy buttons, because these "White Tombac" buttons often come out of the ground looking almost as shiny as the day they were lost, 200 or so years ago.