73 dart seat frames??

Rear bench backs and bottoms did come in serveral different shapes but all fit the same mounting points. You haven't said the cover is smaller than the frame or larger than the frame. Measurements of your frame may serve better than part numbers if you can get other 73 2 door Dart/Valiant owners to measure their frames. I'm curious as to how well the cover fit on the rear seat bottom too. If one piece was changed , chances are they both were.
Lifes situations that would require a seat replaced normally effect both parts. Good luck.