Jimmi,
In 1953-1955, that heater is usually called an
Air Flow Heater and it attaches to the side wall. In earlier years, similar "de luxe" heaters were more commonly called a Fresh Air Heater (look at page 27 in the installation manual that I linked to, above) and those earlier heaters did not have the interior door as found on the 53-55 version (as Jim Carter describes).
The Recirculator (Recirculating) Heater (also shown) attaches to the firewall. And, as I indicated above, this heater changed very little over the years.
