I've never done a 500e seat... but I've done W123 and W124 seats.
It's a good bit of work on a W124 seat. I should say last weekend- I did a new pad for my wife's 420e. NOTE, part number didn't change, but the pad construction did. Thickness is thinner and stiffer.. you had to cutout and re-use the old retaining wire. Lucky I had a full set of retainer clips which were black to replace the white ones which were too short w new pad. Probably needed to add foam layer to the edges so it would fit a little nicer. But it's smooth..and comfy.
There are a the side pads which give lateral restain. If you look on the floor under the seat- is there loose horsehair dust/crumbs? I can tell a difference.. as they get soft with age or big bottoms.
Michael