The "London" in the backmark means it was made in Britain, of course. Indented lettering in a British-made but dug-in-USA brass 1-piece flatbutton's backmark dates it from approximately 1810 through the 1830s. The phrase "Best Quality" is one of ratings in the British-originated button "quality rating system," which referred to the quality and thickness of the gold or silver plating applied to the button.