Other reasons which support Coach Stack's cavary-insignia piece being a civil war era one are the bullets he found with it. I see a .58-caliber muzzleloader 3-groove Minie bullet, several .52 Sharps Rifle/Carbine bullets, a US Watervliet Arsenal bullet for .44 revolvers, a Bartholow's Patent bullet for .44 revolvers, and what appears to be an unfired but damaged .52 Spencer bullet. Among the spent cartridge-casings are several .44 Henry Repeating Rifle casing, distinguished by the "double firing pin" marks on their flat base. The longer casings may be Sharps-&-Hankins casings.
All of those types are civl war era, and those types (except the Henry) quickly fell out of favor a few years after the war's end due to the advent of metallic-cartidges for revolvers, Sharps Rifles, and Springfield Rifles.