That part you are showing is called a "lock washer for axial fixing" in the 603 engine manual. It is used as a sort of thrust washer to lock the camshaft into position so it can't move forwards or backwards.

That is the only part like that which is used in these 603 engines, there may be some sort of slot or recess towards the rear of the head that looks like it might be used for something similar, but there is no part in either the manual or the MB EPC for use in any rear slot.

Part number # A 601 051 00 73 for the part used at the front of the cam.

Mercedes-Benz Teilekatalog (Ersatzteile online)