Looks like a British import flat button from 1820-30 period. The STAND d is an abbreviation type for standard. I cant tell if the lettering is raised or indented.....would make a little difference, CBG can clarify all of this with much better explanations.
Your find is indeed a brass 1-piece "flatbutton," made for use on civilian clothing. (A few non-wealthy local Militia units did use these plain-front flatbuttons.)
The fact that your brass flatbutton's backmark is written in indented lettering means it dates from about 1810 to about 1835. After that time, these plain 1-piece buttons rapidly fell out of favor with the public due to the advent of machinery which could INEXPENSVELY manufacture 2-piece buttons with an "ornate" emblem on the front.
Although the presence of the word "warranted" (which means guaranteed), and the abbreviation "STANDd" suggest British manufacture, American button-makers did use those terms in their backmarks. That being said, the statistical odds favor your flatbutton being a British-made one. That's because American button-makers could not produce enough to supply the demand from the Clothing Industry until about the late-1820s. (Which is why millions were imported from Britain.)