Your excavated US Army "general service" button, a.k.a. eagle-button, was made during the second half of the civil war. Prior to that time, these buttons had to display the maker's or supplier's name/company. But by the middle of the civil war, the yankees were drafting hundreds of thousands of new troops, who needed literally MILLIONS of these "general service" eagle buttons for the uniform coats of privates, corporals, and sergeants. (Officer's buttons had the initial letter of their branch of service in the shield, such as "A" for Artillery.) Apparently, the rush to provide those millions of buttons caused the US Army to accept uniform buttons with just a simple "Quality-rating" backmark (extra quality, superior quality, etc.). After the war's end, there were tens-of-thousands of "leftover" general-service buttons, so none needed to be manufactured... until 1875, when the design of the eagle-with-shield emblem was changed.
