Are Demon and Dart seats different?

What I can tell you from my experience wont answer your original question.
The rear seat bottom was the hardest piece to skin. The instructions say start at the back and work forward. I lay the seat upside down, place a 4 foot piece of 2X6 on the springs and have a couple people stand on it to get the skin to reach over the corners and down the front edge. Tight as a banjo string when done.
Now for what I believe to be the reason... Models with a square back glass can accomodate a taller rear seat bottm than models with a slooped back.
Those models with fold down rear seats would have thinner backs and bottoms. Consider the VW beetle. That rear seat bottom couldn't get thin enough to keep heads off the glass. LOL
Anyway... I think aftermarket patterns for the rear seat bottom skins are "one size fits all" in design. Maybe the factory skins were too. Maybe the frames are all the same and thickness of batting varries for different models.
All the covers I've installed came from Legondary. Never had a problem other than that tight fit at rear bottom. Tight is better than loose in my opinion.
Am I saying make it fit ? No, but first time I encountered this issue and quesioned Legondary (by phone) that's what they told me.