The button's backmark saying "Best Treble Plated" means your find is a brass 1-piece flatbutton which had three layers of silverplate. (In a button backmark, the term "Plated" ALWAYS meant silverplate, and Gilt meant goldplated.) The presence of an American eagle in the backmark means your button was made during or after the War-of-1812, until plain-front flatbuttons fell out of favor with the public in the mid-1830s. Due to the War-of-1812, the US public boycotted British-made goods. Because British-made flatbuttons generally had the British Royal Crown in the backmark, the American eagle replaced the crown to indicate American manufacture.