Here is a picture of the one heater from 1942. That heater was produced by Harrison, interestingly it lived on with Willys Jeeps for several years.
https://www.flickr.com/photos/advance-design_parts_co/8457454850/in/album-72157632724104168/
A GMC version.
https://www.flickr.com/photos/advance-design_parts_co/32593178093/in/album-72157631868452754/
Another detail shot.
https://www.flickr.com/photos/advance-design_parts_co/43790194570/in/album-72157631868452754/
I looked through 1937-1946 Chevrolet Truck Data Books and there is no detail. On the other heater I have never seen one but there were numerous aftermarket heaters available in that time period.
Some more info here and a picture of a complete NOS with all parts (click Heater Super Deluxe).
http://www.1948chevy.com/support/accy.htm