1972 Duster Heater Control Unit - defrost question

Ok, lets take this one step at a time. Go to mymopar and look at the 72 Mopar parts catalog. On page 6 you'll find how to decode the body plate. That will show the date information. Hopefully the body plate will still be on the left inner fender.
You're complete control switch and multi pin rubber vacuum hose connector are from 73-76 Dusters. Very different from the 72 and earlier models.
Pictures of the ac control can be found on the hamtramck registry site. Once there click on library, then under dealership data books click on 72 plymouth image, then valiant/duster. Scroll down and you'll find photo of the ac/heater control switch. Do the same for the 73 model year. There's other useful info on this site.