I have often wondered the same thing. My W116 SD is engine no. 617.950, W123 300D Turbo is 617.952, W126 300SD is 617.951. They all have a different end no., but the same engine manual of 617.95. I think each has slight differences like the later W126 300SD has the cyclonic oil separator in the air filter which the W116 does not have. My '79 300 SD is incredibly fast. The kickdown switch at the base of the fuel pedal is a lot of fun. I hear they are slow, "Super Turtle", but not mine. Maybe it is because it has a new turbo? I also don't have any problem with hills, or lagging off of the line.