Underdash ac vent years. 74 into 72

[lunacy]

I have a 72 Dart with AC, but the underdash vents are missing. I found an assembly out of a 74 online. The part that connects to the duct looks different than what's under the dash in the 72. Did they make all these parts the same or if not, what are the year cutoffs?

 

[RedFish]

The shape of the square opening where this piece joins the main case is different for 73 and later. The louvered all black plastic vent pieces will interchange with the chrome pot metal type but that's about all that interchanges.

 

[lunacy]

The shape of the square opening where this piece joins the main case is different for 73 and later. The louvered all black plastic vent pieces will interchange with the chrome pot metal type but that's about all that interchanges.

What I needed to know. Thank you.