Hi Franco,
I *wish" I would have taken some photos of my seat surgery back in 2011, but alas I did not.
The seat construction, in a nutshell, consists of several items. First you have the seat covering, (tex or leather); the backing; the horsehair pad, the frame, and two kinds of springs. Of course there's mounting and sliding hardware as well. Once you start taking things apart, if you are at all mechanically inclined it all becomes rather obvious.
The first kind of spring you'll find is a fairly common "S Spring"; the second kind are flat bar springs.
Once you get everything apart and take a look, you'll see how all these parts fit and work together. I believe the ends of the S springs are welded to the frame. You may have a case (particularly on the well used driver's seat) where either the spring is broken, or a weld has broken. I suppose in a pure and perfect restoration, you'd want to remove the old and broken and replace with new. An alternative here, easy for most is to "siamese" new spring material onto the old. Instead of welding, you can use a variety of hog rings and crimps (with the appropriate tools) to secure the new springs. The flat springs will not be readily available at upholstery supply houses, as these are sort of custom for the seat.
With the hope of firming up my seat, I took apart the driver's seat. I obtained thicker and more robust S spring material and not only did I siamese this new material on all the springs, I also added two more loops. I added rings and crimps all over the place, significantly firming up all the springing action. However, even after all this, what happened when I sat on the seat springs (with the horsehair pad in place) is that the front of the seat bottomed out, just as it did prior.
That's when I added the foam aka "pool noodle" to the front of the seat and along the sides, which prevented the seat from bottoming out, and significantly damped the springiness of the seat.
I wish I had the photos...but alas, I do not.
