I was reading the manual for my 300D concerning the Allen Bolts that hold the rotor on. The manual relates that they are "Self-locking Hex Socket Bolts". This sounds to me like you will need to get them at the Dealer or a Junk Yard as you are unlikely to find any self locking Metric Allen Bolts at a regular parts place.
See page 2 of 4 and item #9 text and pic.
http://www.pauldrayton.com/uploadfiles/merc/Service/W123/w123CD2/Program/Chassis/42-220.pdf