Back when, heaters were installed by whomever needed a heater, not necessarily the dealer. Lots of aftermarket brands. Since your truck appears modified, the heater being correct should not be an issue. A working heater in your area would be a requirement.
As late as the 1960s, heaters were optional. I know of a 1960 Chevy Impala, top of the line, that had no heater.
1949 F-4 Built to drive--script bed.
1956 F350 Stock rebuilt script bed. Sold
2007 F150 SC. Driven once in a while. 2015 Escape SE EB Wife's ride
Government conceived by genius, now operated by fools.