There may be slight differences on how the rear disc is mounted. For example my '84 300SD (126) doesn't have a allen screw to keep the disc on the hub. It was frozen and took a lot of penetrating oil, pounding on the mounting surface and most important of all...patience. On a '91 420sel that I worked on it was almost the same except it had a 5mm allen screw that held the disc to the hub. Good Luck...