It appears to be a 1-piece brass "flat button" made for use on civilian clothing, and its "depressed" (not "raised" lettering in the backmark means it is from about 1810 to the 1830s. (This type rapidly fell out of popularity in the 1840s, but a few were still being manufactured in that decade.)
I cannot be sure whether the lighter color in the your button backmark's letters is gold or just dirt. Look closely to see whether one of the words "rich" or "gilt" or "color" or "colour" or "plated" is in the backmark. On these early-1800s buttons, gilt means gold-plated, as does rich and color/colour ...and the word "plated" means silver-plated.