I think it's perfectly reasonable that the D and E series docks aren't interchangable...
I mean really, Dell has said that things are compatible across the D series (and c before that) which has stayed stable for six years and god knows how many models so one that you bought six years ago for a 15" workhorse will work for a D420 bought today (same with drives too). This kind of stability in accessories is partly why people buy Lattitudes and the like.
Given this longevity and compatibility that is way over and above what a lot of companies in the industry do I think it's reasonable that every 6 years or so they get to make some changes.
Steve with a sombrero.