Those are covers that go on the bottom inside of the seat vertical backrest supports. You should have two for each seat.
Not if it came off of his 560.....I think they are unique to the SLC seats.
I had to replace some of those on my 560sl. One on the passenger seat, which fit up just fine with the chromed plastic trim.
Another went on the driver seat, on the side with the armrest. For the life of me, couldn't figure out how to slip it in place to match up with the chrome trim. It did not want to match around the metal bracket.
So, it's sitting in my glovebox.