Your find is what collectors of Historical Buttons call a brass 1-piece "flatbutton." It was made from "rolled" brass (meaning, the brass was flattened in a press), instead of being cast in a mold. That type dates from the mid-1700s and almost entirely dies out around 1840. Brass 1-piece flatbuttons with a backmark written in raised lettering date from about 1790 into the 1830s... and indented lettering (seen in your button's backmark) started about 1810. The presence of the word "Imperial" in your backmark means it was manufactured in Britain... like the majority of flatbuttons dug in the US. Although your button COULD be from as early as 1810, the War-of-1812 interrupted imports from Britain, and US customer boycotting against British-made goods lasted into the early-1820s. So, the statistical odds favor your British-made button being manufactured sometime between the mid-1820s to the mid-1830s.