Here's how it is in a W126 -

The upholstey has a hem that fits into a slot just about all around the base. Looking at the seat from below as in your pic, take a putty spreader and gently pry the upholstery away from the seat's bottom panel. A couple off mm is plenty. Then put pressure on the seating surface so there is enough slack to pull the hem out of the slot. The has a ~15mm stiffener that is held in the slot so you have to pull about that much out of the slot. Do this around the base and you'll completely free the upholstery.

Slide the upholstery forward relative to the base and you should have two separate assemblies.

The leater cover will have a burlap skirt stapled to the seat pad. Remove all the staples to separate the leather cover from the pad. But that won't do it because there are also hog rings holding the leather cover to the pad. Straighten the hog ring ends then separate the leather cover from the pad.

Now you have a leather cover with padding held in place by burlap. There is a heck of a lot of stitching holding the foam and burlap to the leather, particularly in the area of the pleats. How you get in there to sew the pleat back together is beyond me. What I was told is that when the stitching starts to come apart, the leather is probably too dry and brittle to hold new stitching. Maybe leatherique can bring it back to life.

FWIW I was able to pull the leather cover and pad off a W126 front seat without pulling the big heavy seat from the car. I have no idea if I can get the cover back on with the seat still in place.

I have some pics at http://photos.yahoo.com/tabijan in the SDL front seat album.

Sixto
95 S420
87 300SDL