One of three (3) cartridge packs for Army Pistol (Colt).
B - Wood block wrapped in paper, printed, "6 CARTRIDGES / COLTS ARMY PISTOL / C. S. LABORATORY / RICHMOND VA". Handwritten between these lines is illegible text, and possibly "Mar 1884"
One box (a) of cartridges came from the last rounds of cartridges issued to Mosby's men in 1864.
The history on the other two boxes of cartridges (b, c) may or may not share this provenance.