I'm no Leupold historian but I've owned a bunch of these 6X42 Leupolds.
The M8's were the 'father' of all the versions that came after. Actually, M8 designated a fixed power scope no matter the magnification. I believe when they changed to the click style 1/4" w/e adjustments the desination changed. Early versions did not have the MultiCoat lens coatings. There were the M8 based 'Tactical' 6X scopes with an a.o. and 1/4" click target adjustments as well as the current FXIII 6 power a.o. Hunter Benchrest scope with 1/8" adjustments. Before the 1/8" adjustment models were in production, Allen Tucker from Leupold arranged for the retrofitting of one of my earlier 1/4" adjustment models to the prototype 1/8" adjustments and I wrung it out in competition a full year before they were commonly available.
I consider 'em all M8's when you get right down to it...some have more gingerbread on 'em, is all.
The older friction adjustment scopes hold their p.o.a. very, very well. One BR shooter that I know had his 'click' w/e assys. replaced with friction adjustments and has won two national Championships with his 'back to the future' scope.