It's important that your studded 2.25" shell has a flat base, instead of a "rounded-edge" base. If the studs are copperbrass, it is an Armstrong type, from the 1860s through 1880s. (I should mention, no 2.25"-caliber Armstrong projectiles made it across the Atlantic Ocean during the civil war.) It the studs are lead or a similar "White-Metal" alloy it is French. (But none of those made it across the Atlantic during the civil war.) The similar-looking Spanish-American war one, which had lead/White-Metal studs and was made in Spain, had a rounded-edge base.