This is the Evans Underchassis heater that I purchased in NOS condition back in the 1980s from Surplus City Jeep Parts. It is one of the “Interim” or “Quickie” kits mentioned in the WW2 Army Motors and not really expected to be used on a jeep.
The idea was that you would place the heater under the engine and then drap a tarp around the vehicle to make an enclosure. The heat would keep the engine warm enough to start at very low temperatures. Even though I lived in Alaska, I never had the nerve to give this a go. I felt like it was NEVER cold enough.