Made in Britain between approximately 1810 and the early-1830s, imported for American use because the fledgling US button-making industry was unable to fulfill the demand for brass buttons by the fast-growing American civilian clothing industry.
The backmark's words "IMPERIAL STANDd" is an abbreviation for one of the quality-ratings in the British button-making industry, referring to Imperial Standard. Examples of other British quality-ratings are Extra Quality, Superior Quality, Superfine, etc.