The “Franken” heater shown is exactly that. That heater how ever it came to be is incorrect. For 1954 there were two accessory heaters available. Fresh Air, called Air-Flow and Recirculating. The additional heater you purchased is also a correct Air-Flow but not a 1953 or 54 model. The heater shown also has the plaque instead of a decal, this heater was offered from 1947-1950 it may show up on a few 1951’s until stock was consumed. The two heater types are shown below.
Here is a picture of of installed type in a ‘54 GMC.
https://www.flickr.com/photos/advance-design_parts_co/25248149898/in/album-72157631868452754/https://www.flickr.com/photos/advance-design_parts_co/8129448404/in/album-72157631868452754/This is the correct heater w/door to dump heat into the cab. You will note cable to door, manual lever to defrost.
https://www.flickr.com/photos/advance-design_parts_co/8129313579/in/album-72157631868452754/https://www.flickr.com/photos/advance-design_parts_co/8129470099/in/album-72157631868452754/The switch is black, usually Ark-less or Delco-Remy, they are flat to allow Fog switches to be right next to them on the dash, earlier versions were this way to allow Heat/Defrost to fit the same way, the dash is punched for them. Lighted or rotary switches while very common are not correct. Photo below shows the design element of side/side.
https://www.flickr.com/photos/advance-design_parts_co/46016562265/in/album-72157631868452754/Correct switch type.
https://www.flickr.com/photos/advance-design_parts_co/8129352382/in/album-72157631868452754/The knob for the door cable is shaped like your “B” knob but has a white dot in the center as shown.
https://www.flickr.com/photos/advance-design_parts_co/8129338276/in/album-72157631868452754/
