Before the days of Carbon, what was reckoned to be the best section ali mast for lightweights??

Pre Super Spars, the best ali mast for light weights was probably the Proctor "C", with the "D" being best for medium weights and then the "B" for those of a slightly fuller figure! Not sure about the later Proctor sections such as the Alpha and Beta minuswhich came to prominance when the Merlin was sailed by heavyweights.
When Super Spar became the market leader, I think the lightweight was the M1 and for heavyweights the M7

I think Chris Rhodes did a lot of the work on trying to make an extra light section work.....  destroying a number of masts along the way. The Proctor C that he used was really too light, but Chris sleeved it in the high stress areas and finally developed a rig that took him to a Nationals win (at Falmouth??)

Otherwise (if you were a Proctor man) it was a D or a Beta (+ or minus, depended on the crew weight)

D

I had two successful boats with Proctor 'C'masts - 1047 a Proctor MK XI and 1502 the first Adur 7. I think most of my subsequent boats had 'D's in which I was never as successful, or maybe it was just me!

See the photo of 1047's 'C' mast under stress in the vintage photo section.