Up front and hard to miss is Angus Young, the diminutive dynamo of a lead guitarist, wearing the sweat-soaked remains of a velvet schoolboy uniform, duck-walking and thrashing his head like the lightning-strike victim on the cover of “Powerage.” Nearby, prancing bare-chested, is the lewd and mischievous lead singer, Bon Scott. (He’ll be dead by the end of the decade.) But, if you can take your eyes off these two showmen for a moment, you might find your gaze drifting to the left of the drum riser, where a pugnacious long-haired kid (he looks like he’s still in high school), wearing jeans and a white T-shirt, is strumming his Gretsch guitar and shaking his leg in time to the driving beat. His name is Malcolm Young, and you could be forgiven for seeing him as just another part of the backing band, but he is in fact the mastermind of the whole operation, at once its visionary and its taskmaster. He is the soul of the band, its leader on and off the stage.