US British Import M1860 Sergeants Enfield Cartridge Box
These boxes were imported from England though I have seen very few that could be associated to the Union Army.
Maybe they are only issued to sergeants but there is no evidence to back that up.
The enfield sergeants box were to have a tool pouch mounted along the top of the inner flap and a percussion cap pouch mounted on the side and have been stamped with the British War Department Stamp, they were issued with a black leather sling unlike the CS issue.
Available on back-order