Your find is a US Army uniform button manufactured between 1854 and 1874 for use by Enlisted-men's ranks (sergeant, corporal, private). The eagle had a different shape before 1854 and after 1874, which enables us to time-date your button to that specific 20-year period. However, instead of having the manufacturer's name as its backmark, it says simply SUPERIOR * QUALITY * in "serifed" lettering. That specific backmark is most often seen on civil war era eagle-buttons, when the US Army needed so many millions of them, the peacetime requirement for having the maker/dealer's name as the backmark seems to have been temporarily suspended.
If you are wondering how your civil war era US Army enlisted-man's uniform button came to be lost in Massachusetts, the answer is, every state's troops were issued their uniforms and underwent some "in the field" training IN THAT STATE before getting shipped of to combat locations in the Confederacy. For example, more than a few civil war Military relics have been dug at Mustering-In encampment sites in states like Iowa and Wisconsin, which never saw any civil war combat action. You might be digging at or near a Mustering-In encampment in Massachusetts.
