Victorian Era Metal Sandwich Case from Crimea War?

[GCCE1854]

Just ran across this article in the National Army Collection. It's listed as "Sandwich case used by Lieutenant-Colonel Frederick Francis Maude, VC". 

There's basically no info at all about the piece itself, except that it was not part of the army-issued canteens and was privately purchased. When I try to look up a "sandwich case" in the Crimean War, mentions of this one are all that come up. What would he have used this for (was it to carry food?) and where would a soldier have carried it?