Can’t comment on tea pots, but when I was Canadian infantry, '81-'85, I was an M113 driver for a year. A water heater was located just behind the driver, squarish and light grey like your first example.
Ours had a spigot for dispensing hot water, and the top opened to put boil-in-the-foil MREs for heating.
One very important driver responsibility was to always have hot water available. Hell was to be paid if the rest of my section returned from pounding the ground and the water wasn’t hot for coffee. Or tea.
