Accessory heater types, Recirculating and Air-Flow look like this, these would be the two available types purchased at the parts counter in 1951.
https://www.flickr.com/photos/advance-design_parts_co/8129448404/in/album-72157631868452754/
Recirculating firewall, cover plate will be in body color or black.
https://www.flickr.com/photos/advance-design_parts_co/8129182431/in/album-72157631867396207/
Air-Flow firewall with Norco valve.
https://www.flickr.com/photos/advance-design_parts_co/8130478168/in/album-72157631870734114/