The 53-54 heater has a door to route the heat into the cab.
http://www.flickr.com/photos/advance-design_parts_co/8129312977/in/set-72157631868452754
The earlier version does not as shown in this picture of a NOS unit.
http://www.flickr.com/photos/advance-design_parts_co/8129439492/in/set-72157631868452754
I am unsure on the core fit? If there is a difference it must be because of the door.