Note that the rosette is a thin sheetbrass disc with its edges folded over to retain the thin IRON sheetmetal back. Also, the "legs" of the iron crossbar pass through the disc to its front, where they are peened over, like a rivet. I've never seen a civil war (or earlier) rosette with those characteristics. Iron-back horse harness rosettes are considered to be post-civil. For example, the one with a raised US mark and iron back is from the Indian Wars era, onward into the 20th Century.