I am not an expert, but everyone that I have taken apart has had a wire or a small rod in the bottom pocket, which the hog rings go around?
Yes. I watched the legendary videos and fished their paper coated listing wire around the cover. Then put a shorter piece on the back side so the hog rings had something to grab. The old covers just had a small rusted wire inside which was a little thinner than the paper coated listing wire in the legendary assembly kits. also used the same paper coated listing wire for the center portion to pull down into the bottom and back portions of the seat covers.
For that location, the seat covers have a wire that is wrapped in paper. You take them out of the original seat covers and put them back in the new covers. Duane
I use 1/8" welding rod in all of the channels, with a 90* bend or loop at the ends so they don't work their way out and puncture the cover.