Bill,
Those 1955 heaters are "Accessories" listed under the overall title "Options" (at your "Optional Accessories" link).
It looks like the RPO numbers (part number suffix) you are showing are for Canadian numbers/options/codes? USA Option numbers do not use the part numbers (from the parts catalog). The USA accessories are listed as part numbers.
Thanks for the information about 1957 heaters. That "dealer installed style" is not a USA truck Advance-Design heater. Might be a 47-55 Canadian Chevrolet truck and/or a GMC style heater.
USA Chevrolet Truck info is given below:
Option numbers (not part numbers) are shown in USA Truck Data Books in 1946 but not in 1941. Those option numbers were listed as "OPTIONAL EQUIPMENT" in the 1946, 1947, and 1949 TDBs and are listed as "REGULAR PRODUCTION OPTIONS" in 1951-1956.
