Brass 1-piece "flatbuttons" first start showing up with a RAISED-lettering backmark (like yours) about 1790. So, I'd add 20 years onto the front end of the 1810-to-1840 timeperiod FreeBirdTim gave you. I should mention, his estimate of 1810-to-1840 is correct for brass 1-piece flatbuttons with INDENTED lettering.
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