In Tice's book that Infantry Eagle button is GI215A39. There are many variations, including a few from this same maker, but this one has the "rays" between the center and the lettering. According to the Tice book, this is a pre-1866 backmark. I honestly don't know if that means it's ONLY pre-1866 for military buttons, or was used both before and after 1866. According to the link below, that backmark was in use from the 1850s until 1877.