Having a Hagon Jap, 2V Jawa longtracker and Mk 1 Godden Wslake, I would see that classification should reflect tje transitions of the sport. Pre 70 which allows for many of the home-made specials and then 2 valvers and 4 valvers with the next wave being the modern laydowns. Probably far too basic but hopefully a start. I will mow await the tirades which seems to happen these days,
Rosco86