Thanks, Achim. You are correct -- when I bought by car in Germany in the late 1990s, I wanted to put on new seat covers, but only the later material was available. In the end, because MB Tex basically never wears out, I just redid the springs and pads in the seats and they look almost new.
I can tell that even the Classic Center had a problem when they restored a late 230SL. They also used the newer material to restore it (and wanted $230K!).