I would try the rear set of hooks first. My convertible only has one set, the front ones. If I install the seat the proper way, meaning inside the hook, the seat bottom is too far from the seat back. There is a huge gap where they meet. To make it look decent I have to install the seat bottom so that it's behind the hook/bracket. I think my car was suppose to have the rear set of hooks too. Someone dropped the ball at the plant. Also the build sheet I found (back in 1980) was coded for a hardtop, maybe that had something to do with the missing brackets.
I emailed the person that took those pictures and asked if they know anything about the missing hook.