IMO, the Dura-Ace AX group is one of the most under-rated of all time. It's often the subject of disparaging remarks, due to the number of technical innovations it introduced. I've always found this ironic, given it's clear influence on C-Record and the fact that C-Record was even more idiosyncratic. My favourite feature was the Dyna-Drive crankarm and pedals (which had previously been introduced on 1980 Dura-Ace EX). I was disappointed that hey never became more popular, though the AX series left its marque on cycling by popularizing aero seat posts, aero brake levers, aero stems and aero styling in general.
Dura-Ace AX was 7 speed only, right back to it's introduction for the 1981 model year, so there's no dating guide other than the component date codes or frame S/N.