The percussion cap box/pouch was used by the US Military as a revolver cartridge storage place for many years after the ACW.
There were a lot of them around. No longer needing percussion caps, these boxes served this need very well.
Revolver ammo was being packed 12 rounds to the box by this time. 6 went into the revolver and 6 went into the box if needed.
I have found that 12 rounds of .45 Colt will fit into the Civil War percussion cap box quite well.
For use within the Military, the Musket vent pick, along with the wool fleece were removed from the Civil War cap box/pouch for this usage.
Some civilians may use this arrangement as well, early in the period, With the introduction of the cartridge loops on belts, one will seldom see the cap box used by civilians.
My best,
Blair