Yes, it does appear to be a cartridge-box finial... but the finder (CKaufman) did not tell us what the metal is. If it is brass, it could be from either yankee or Confederate OR from an Indian-Wars (1870s-1890s) cartridge-box.
Most Confederate finials were brass, but they did manufacture a few from pewter and a few from "hardened-lead" -- which is a solder-like alloy.
So, CKaufman, is your cartridge-box finial made of brass, or of pewter/hardened-lead?