Just speaking up in confirmation of what Filmiracle and DCMatt said... from World War One through 1923, the US Army "Great Seal" buttons (and US Marines buttons) for Enlisted-men's ranks (sergeant, corporal, and private) were manufactured with a "black finish" on them, whose purpose was a sort of camoflage, to prevent a shiny brass button from revealing a soldier's postion to the enemy. Your button clearly shows some remnants of the "black finish" on the brass button's body, thus dating it to the World War One era or slightly later. However, because production of those buttons declined drastically after the war's end, your button was most probably manufactured during the war.
