I just have no idea, this one has been bugging me all day. thinking of how it was used, and it seems counterproductive as a buckle with any kind of fine adjustment. my reasoning it that there would have to be sizable slots in the leather for that loop to fit through. hope one of the old timers has seen this before and can identify it for ya. ill keep looking in my free time though
It is a horse-harness strap "locking buckle." Here's a photo of a similar one. We don't dig them at "pure" civil war sites, so they are from the late-1800s onward.