Thanks to everyone for the comments, and in particular thanks to 97 SL320 for leading me down the path to enlightenment.


It seems the crank journals and con rod caps were subjected to an Induction Hardening process. This particular OM 603 is from a British Army BV 206 which has been through the Army Base Repair system, so I'm not sure if the process was done then or whether all OM 603's are like this straight from Mercedes.


As for the rod movement, 97 SL320 is correct, these engines have piston guided rods.
Interesting stuff, I enjoyed reading up on the subject. :-)