This cover plate is the same and in the same location of any 48-52 right upper fender. Pete, look under the front edge of the right fender, above the right headlight. That's where this plate is located. If your's is missing, I may have one.
OK . . . sorry. My bad . . . the fender does have the mount incorporated. Thanks Ilya!
Anyhow HH we all now know what the plate if for . . . your original question.
Mervy, my 48 didn't have a heater either. I still have all the hose knockouts in the firewall, along with the square knockout for the fresh air vent in the kick panel. I understand the fresh air was an option in addition to the heater.