It doesn’t resemble the one shown in the 1946 Accessories Data Book or the 1946 Chevrolet Truck Data Book. They each show one with two horizontal louvers, each with a knob and on the bottom of each louvre and a Chevrolet label with the BowTie at the unit bottom. Attached are some photos of the pages from one of the books referenced.
This is an aftermarket heater. Back in the day, people would install anything they could find and fit to use for heat. There was no "factory installed" heaters. All were aftermarket brands or dealer installed Harrison brand heaters. Here is a photo of the correct heater for 1946 as shown in the 1946 Salesman's Truck Data Handbook.[u][/u]