Is the Plymouth Duster a fastback?

I don't believe it is. Because there is no notchback option? I understand that's not always true for all fastbacks of all dates and size. But to me. A fastback suggests there are non fastback options.

The "notchback option" with respect to the Duster was the Valiant or Scamp.

The 2 door notchback option to the Demon or Dart Sport was the 21 or 23 body Dart or Swinger.

A "29" body can be a fastback or a rear window modification as mentioned above on the Challenger SE.

72-74 Chargers were 21 or 23 body cars except the SE which was a 29 body.
Same roofline but radically different rear side window treatment.