Are you sure you didn't find info in the archives under "seat" or "seat rail", etc.?
I believe the early W210's had problems with loose seat rails, and remember reading a number of posts on this. It is not an uncommon complaint, and comes up on the board from time to time.
In any event, the solution seems to always be to replace the seat rails.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".