The cushions on the bottom of the stack have wooden undersides. I'm not really sure whether these are the seats or the backs, though I think they are the backs. The seats, it seems to me, are the thinner ones on top.
The cording on the edges of the cushions was my least favorite part. I removed the cotton cords from inside the original material and resheathed them with the new fabric. That takes time!
This is the wooden side of the taller cushions. The original fabric being vinyl, it was just nailed down. But an unfinished edge of this material would have frayed, so I cut these out with a larger seam allowance than the originals and tucked about a half inch under before nailing them down with upholstery tacks.