The original Chevrolet heaters from the '30's and '40's were made by Harrison Radiator, and were installed by the dealers and not at the factory. The knockouts were stamped in the firewall at the factory, but the heaters were installed later at the dealer if the customer wanted one. Many times, the customer would buy the truck without any options, and then return to the dealer in a year or two when it was paid for to get the extras. In the pic InMy40s linked to, this is a 1941 Chev. Deluxe heater in a 1940 truck. May have been that the truck was not sold until 1941 also, and that's why it ended up with a '41 heater. That's what the dealer would've had on the shelf in 1941.
S10's pic with the angled doors is a correct 1940 Chev. Deluxe heater. 1940 heaters had the angled vent doors. Earlier and later heaters have straight (horizontal) doors. Chevrolet heaters in the late '30's/early '40's came in Deluxe and Super Deluxe versions. The Super Deluxe was slightly larger (more heating capacity) and had fancier trim. It also cost more. Both heaters were available with an optional defroster blower mounted on top to blow warm air to the windshield defroster vents in the dash. If your heater does not have the defroster blower mounted on top, it will have a block off plate held on with 2 screws where you can easily add one.
Tiny's heater pic may have been taken from a '40 truck, but is actually a universal Chevrolet heater that dealers sold in the later '40's. I have seen those in '48-'49 AD trucks before. Also, many companies made aftermarket heaters in the '30's and '40's. If it doesn't have a Chevrolet emblem on the front, a Harrison tag on the back, and the pipes don't fit the factory knockout holes, it's not an original Chevrolet heater for your truck.
Pics of original 1940 heaters, defrosters and other cool stuff won't be found anywhere other than the 1940 Accessories Book. These are small catalogs, maybe 30 pages long or so, that show all the accessories and options available from the dealer each year for cars and trucks. Reprints are often found on ebay or from dealers (Carter's maybe?) for $10-$15, and are tons of fun to look at.
