Hi Huddie
The fulcrum pin is the common pivot to both shoes, opposite from the operating cam.
Not much of an update at the moment as the bike has had a holiday whilst the magneto is being overhauled. When all is back together, I'll update the brake situation. For what it's worth, I slackened the fulcrum pin, pulled the brakes on really tightly then tightened everything, but the brakes were very little better. I expect that the linings are quite old & could do with replacing. Meanwhile, the rear is about the most powerfull I have ever had on a classic!