The item shown is item M15753 in the Horse Soldier catalog and is reputed to be from a Massachusetts collection. the box bears the name of "E.W Emerson" roughly scratched onto the inner surface of the outter flap.
It is fitted to accommodate the .52 caliber cartridge and measures 6 ½" wide by 5" high by 1 ½" deep.
The outer flap of this box has small vertical slits to accommodate the brass box plate but no plates were ever issued with these boxes. [HS remark] Latch tab secures unto a round, ball-type finial attached to the bottom of the box. Two vertical leather loops are sewn and riveted
Box has its two, original, japanned roller buckles stitched to the bottom.
Two tins, each with 20 cylindrical tubes and space below for a packet of 10 cartridges allowing the box to accommodate 40 rounds of .52 caliber ammunition.
This, gentlemen, is the cartridge box we should be wearing unless you're carrying a Burnside.