The M Pump is rare on W123s because it was only available on european cars. However, it doesn't offer and advantages that I know of. I just put a european 617 engine in my 300CD and it honestly isn't anything spectacular, other than is lacks all the emissions stuff. From what I've read the motor itself is off higher compression, taking advantage of Europe's higher-cetane rated diesel (can't confirm). Performance-wise I can't really say...