There is a little information about
WWII truck heaters on page 54. There is no mention of a truck heater option or accessory in the 1941 Chevrolet Truck Data Book.
On page 55 of the link above is information about the new Harrison truck heater/defroster made for the new 1947 Chevrolet trucks
"The most complete ventilating unit ever offered to truck operators", Option No. 385 Fresh-Air Ventilator, Heater and Defroster.
Do you have a photo of a 1941 GM truck heater? You mention a "triangular type" heater. A photo might help people find information on that particular heater (are the photos above the triangular heater)?
Since Harrison was a part of GM, and Harrison made inside-air and outside-air heaters for other GM vehicles, maybe there is information on 1941 (or WWII) GMC heaters somewhere?
"The dirty secret is that the military trucks are pretty much direct descendants of the 41-42 civvy trucks (the first batches had civvy gauges and still sported chrome)." This widely known fact is hardly a secret; and, calling it dirty seems an odd exaggeration.
A little more historical information is found in this document. This style of Deluxe Heater Defroster was supposedly available from 1941-1946, and it is also listed as an accessory heater for 1947 Advance-Design trucks.
Jim Carter shows this style heater and suggests (without that the heater unit and the defroster unit were separately ordered items.
Here are photos of a common heater for 1941 Chevrolet cars:
Front of heater and
Rear of heater. People have posted that they have found this style of heater in their trucks. There is a reproduction 1941 car/truck accessory brochure that supposedly has photos - maybe it shows heaters?
