OK.....which heater do we have and what is the correct nomenclature.

1954 Chevy 3100 truck....do we have a Harrison fresh air heater...there is a knob under the dash with a cable pull the knob a little door opens on the heater box. We were told that this is a GMC heater and we needed a round type? As far as we can tell this heater assembly is orginal to the truck. We are going to have the heater core rebuilt but would like to know what we have and what is the correct nomenclature. What the heck is that little door all about? My wife sets the temp in her Lexus and it's all automatic.....no little door with a cable.....