Based on 40+ years of being a civil war relic digger & dealer, and having very diligently observed the characteristics of the metal objects the armies used at that time... that buckle is from no earlier than the late-1800s. Note the two ridges at the center of the buckle's tongue bar, whose purpose is to keep the tongue from sliding off-center on the bar. No buckle from the American Civil War era that I've ever seen has those tongue-centering ridges. But they become commonplace around 1900.