Why are you telling us it is made of pewter? We know that in actuality it is made of zinc. Here's a tip, intended to help you learn to tell the difference between those two metals. "Antique" pewter tended to contain some lead... therefore, it feels somewhat heavy for its size. (Modern-era pewter contains no lead, for Health/Safety reasons.)
Your find's verifiably-correct identification is, a 4-hole zinc pants-fly-closure button from a World War One era US Army uniform's pants. Those buttons came in two sizes, with somewhat different markings on them, in "raised" lettering. The larger size was for pants-waist closure, and it was marked "U.S. * ARMY *" (note the location of the two stars). The smaller version was for closing the pants fly, and was marked simply "*U.S.A.*". The fact that you can see the A (which isn't part of "ARMY") on yours means your found the pants-fly closure version. See the photos below.