As posted by Red, heaters were accessories until sometime in the late 50s. There was no standard heater installed in trucks up through at least 1956. However, the heater was an accessory at least through 1956 (a dealer or owner installed accessory). I do not have 57-59 documents.
But, in the 1960 Truck Data Book, a heater was listed as a factory installed option: RPO-116 Recirculating heater; or, RPO-101 De Luxe heater (and, of course a heater could be installed in a standard heater-less 1960 truck - installed by the dealer or by the owner).
Original/reproduction GM accessory lists are available from at least 1947 onwards. And, the Truck Data Books list and described the options that could be ordered from the factory.
Many factory (factory-installed) options are listed as being available on Chevrolet trucks in 1941 (and in 1946), but not listed in 1939 (at that time, it looks like you bought a truck and that was it).
Of course, dealer-installed and owner-installed accessories (made by GM and by other companies) could be obtained in any year (to add "value" and features for the customer, and to add $$$ for the dealer).
