The two "factory" heaters for the 53 are these. Only difference is the 53-55 1st Air Flow has a door like the one in picture 2. Round core is difficult to source, square core is now reproduced. All levels of these heaters are available from parts to restored to NOS.
https://www.flickr.com/photos/advance-design_parts_co/8129448404/in/set-72157631868452754
https://www.flickr.com/photos/advance-design_parts_co/8129470099/in/set-72157631868452754