OK, very good, have some place to start!
Unfortunately, the Feds had very few Arsenal Pack Labels, usually they marked the caliber on the ammo crates the arsenal packs came in, using plain wrappers for the AP's. BUT...St. Louis Arsenal used labels on their AP's through the war. See attached file.
I have reproduced that particular label, be glad to forward it to ya. Also could forward you instructions on how to make nice looking blank cartridges you can form yourself, for arsenal pack use like they did back then.
Kevin Dally