This sort of back-pack food thermos has been around since before the First World War. It's much easier for one person to carry a squad's soup/stew ration in one large container, than to separate it all in each soldier's mess kit.
These are built tough, and are designed to keep hot food warm, and cold food cold in field environments. Holds about 10 Liters of liquid (soup or stew) and will last a long time.
These are really hard to find, and are a must-have for the reenactor or outdoor enthusiast!