Alloy heads on a 617? and other engine lineage questions

[The Warden]

Hello! I had always been under the impression that the 616 and 617 were very similar engines, with the major change being an extra cylinder (and all of the space issues and crank issues that come with it), and that the 603 was a ground-up new design, and that most MB diesels built after the 603 were based on it (the 2.2l, 2.5l {did the 190D 2.5l and the 300D 2.5l use the same engine?}, and 3.5l engines come to mind).

Now someone on another site's saying that there were 616 and 617 engines built with an alloy head (I'm guessing he means aluminum?), and that the 603 and later engines were based on the 617. Does this make sense? Also, how does the new CDi engine fit into this?

Thanks!