When I was in the army in 1977 (!) we used to have similar items when out camping for the weekend, plus 'hayboxes', insulated boxes for larger quantities of food. There would be three individual cannisters inside, as not_an_eloi has said. They were used to transport the hot food but not keep it hot for any great length of time. Just cook it, put it in the cases, transport it out and eat it.

it was so bloody freezing we would have eaten anything hot, if I recall correctly. 😉