The fat round wire that is sewn into the perimeter of the canvas can be seen in this pic:

- Type IV or Type V Bottom view.jpg (206.48 KiB) Viewed 108 times
The flat steel bandings that helps keep the shape of the perimeter wire and hold the canvas can be seen in this pic from Mark's MZ Radio jeep:

- MB MZ Rear Seat Mark T..jpg (116.93 KiB) Viewed 108 times
John Bizal's passenger seat:

- Bizal Pass Seat S Spring.jpg (108.21 KiB) Viewed 108 times
Atlas Royersford and Universal are two of the mfgers and there may be more who supplied Ford & Willys....
The steel banding is about 1/4" wide and almost 1/8" thick I believe....I don't have any "S" types here to check for you at the moment.
PS...note that the construction of the #1 pic is different from the #2 and #3.....I think #1 is perhaps Ford and the other two Willys, but nothing definitive to confirm that.
.....then we have the COIL type springs to try to fit into the scheme of things!