EPC shows the
601 015 06 01 is used on "early" engines and would be correct for any 1980's vintage OM60x engine. If you want a straight bolt-on with no hassles, get this one. (If you haven't checked the price, make sure you are sitting down.)
The
604 015 01 01 was phased in on all OM60x engines later in production, including the 603.960 as of #0049509. Probably around mid-1990's. There is no .961 or .970 engine break as the W126 was out of production by the time the new cover was phased in.
As to what the difference is... if you poke around the EPC enough, on European engines where both the 601 and 604 cover are listed on the same engine, the change appears to correspond with the new rotary vacuum pump. I suspect if you get the 604 cover, you must also get the full late vacuum pump setup, including the drive sprocket/gear/whatever on the IP. Wild guess: the 604 cover may lack provisions to properly lubricate the early vacuum pump? Really don't know, sorry...
