Fender Tag Confusion

I just decoded the fender tag of my 72 Duster and it says it that the car was equipped with a/c and heater (H51). But this car never had a/c or heater. Its been in my neighborhood since it was new and never had it. Has anyone else come across this situation? I've had the car for over 20 years and got it from my friend who live a few houses away. He got it from his friends parents who live about 5 blocks away. It also has the heater delete plate on the firewall and on the dash. So I've been around this car basically since it was new and it was always a heater delete car. Just trying to figure out WTF.....