In the bottom view picture, this looks like the "old" arctic heater setup. The heater itself was mounted in the commander´s footwell. The exhaust could then either be diverted to that oil pan cover, or could be exhausted to the right in front of the commander´s footwell. (You see that exhaust tube in the picture on the left).
I only saw this on USMC trucks operating in Norway, but that might be due to my location in Europe....
I have a few parts of that "old" arctic heater kit, but am looking for the rest. So in case you want to get rid of all that unneccessary weight, just give me a note...