I assume you mean the 1-piece brass flatbutton in the middle of the three buttons, which has a stamped/pressed raised (not indented or engraved) emblem on its front. It can date anywhere from the latter-1700s onward into the 1830s. It's unlikely to be from any later than that, due to the advent of inexpensive ornate 2-piece brass buttons, as I indicated in my previous reply.