The back of the buckle's narrow end has two flanges (ridges), which kept the buckle's tongue centered on the bar. That characteristic is not seen on mid-1800s (or earlier) buckles that are shaped like yours... so I believe it dates anytime from the late-1800s through the 20th Century. But because Civilian-usage buckles are not my specialty in relic-study, I of course could be wrong, and if so, I'm sure somebody here will post a correction.