I think what you are referring to are the different styles of headstocks from the 80s on? One style, referred to by Hans as the "center raised" and Beesley as the "domed" style was introduced ~'64 (according to Hans' book) and superceded the "open book" or "lip" headstock. The other looks similar to the domed, but tapers more towards the top so the top is narrower than the base of the headstock. Beesely calls this the "tapered" headstock... and looking thru his "catalog" it appears to have been introduced sometime in the early-mid 80s. I can't quite make rhyme or reason of who got what headstock... it appears most of the low-mid range models (like the D-50) switched headstocks in the mid-80s from domed to tapered. The D-55s have domed throughout, D-60/62/64/66 tapered, D-70/80/100 domed, DV-52/62/72 are all tapered. If there is a pattern, it isn't really obvious to me... Hans?