Note that this buckle's center-bar has a small flange (projection) on each side of the tongue. These flanges' purpose is to keep the tongue well-centered while somebody tries to use the buckle "under rough working conditions."
Those "centering flanges" start showing up in the very late 1800s. I've never seen them on any civil war era buckle, neither Military nor Civilian. They became quite common by the mid-1900s.
Due to the obvious flimsiness of the (missing) attachment points (hooks/loops/bar, etc.), I'd have to say it is a Civilian buckle