Maybe it's the Attachment Style that makes yours Date to that time period
I have dug a couple Iron Back Rosette's in a Calvary Camp I found 'late War', they had very Thin Brass Fronts that were 70% Gone one looked like a Bulls-eye Front & the other I could see to tips of a Star . Def , a U.S. A-lot of Horse Gear, Mostly Iron, & Saddle Pieces & lots of Dropped 52 cal cone based Sharps , Burnside's, & Spencer Carbine Bullets .
IMO I may be wrong but the US on the op's post looks post war "Maybe"..."Maybe".
Davers