It is a World War One US Army gasmask bag strap's buckle. You can see it at the end of the strap in the photos below. To use T-Net's "enlarge the photo" option to see that the buckle matches yours exactly, click on the photo, then click it again. One side of the bag has the soldier's identification and the battles he carried it in. Those markings confirm the World War One issuance of the gasmask bag with that strap-buckle.
I should mention... that buckle might also have been used on other soldier-equipment straps during that time-period, but this is the only equipment I've personally seen one on.