It is an aftermarket heater (not sure exactly which brand). There were numerous options available back then, Firestone, B.F. Goodrich, Arvin, Montgomery-Wards (Sears), etc.
https://www.flickr.com/photos/advance-design_parts_co/8377841188/in/album-72157631868452754/
The two standard heaters available from GM in '54 were versions of these.
https://www.flickr.com/photos/advance-design_parts_co/8129448404/in/album-72157631868452754/
The 53-55 1st Series Air-Flow differs in that they have a door as shown here.
https://www.flickr.com/photos/advance-design_parts_co/8129313579/in/album-72157631868452754/