Does this earlier thread answer it for you. I think I copied it correctly but if not, just go back 2 pages of posts under convertibles and you should find it. If not, hopefully someone will post with another photo.
After the upper lean back and the side armrests are in place, the bottom is pushed back. There are 2 loops of stiff wire that protrude from the bottom of the seat. These are pushed into an oval cut out in the floor reinforcement. If the seat is not tight in the back, you can push the seat further back so the loop is BEHIND THE REINFORCEMENT. You can see the indent the metal wire has made in the reinforcement on this original 57 convertible.
FYI, the same method is used on all the 2 door hardtops.
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