Bucket seat differences

[The _King!]

I've got two front seats out of my 1970 Challenger. When I removed the trashed covers and seat buns I noticed that the two seat back frames were different. One was sprung and the other was not. The one without the spring section looks like it has supports in the seat foam itself. Were the seat designs changed at some time?

 

[challenger6pak]

Post pics of what you have.