The button's body does look like a late-1700s/early-1800s brass 1-piece "flatbutton." But in the only photo, that square shank looks like a 20th-Century shank. Please clean the dirt (or whatever) out of the hole through the shank, and post at least two well-focused closeup photos showing the shank from different angles. That will help us determine with certainty whether it''s a 200-year-old button or a modern era one.