Your find is a US Navy uniform button, manufactured sometime between 1798 and mid-1813 (when the shield was removed from the emblem). Yours is from the earlier end of that range, because the oval-shaped shield precedes the spade-shape shield. The use of Old English lettering in the backmark strongly indicates it was made in Britain.
Unfortunately, the Albert button-book shows very-very few cuff buttons, so I was unable to find an exact matchup for your cuff button in that book.