I'm guessing that because Plymouth was the low cost brand, the scoops and such were reserved for the "upscale" brand (Dodge). Too bad because the twin scoops look so good on the Duster.
For that matter, I just don't understand why Mopar never put the AirGrabber hood on a Duster. The one on my brother's old Duster has to prove that it would have been a hit. If things ever improve, I'm going to find a way to duplicate that hood.