My opinion only: By 1968, the only use for M2 ball was in putting together tracer linked machine gun belts. The US military had no need for "down loaded" .30-06 ammo. A few thousand M1903's & variants had been sold by the Director of Civilian Marksmanship in the early '60's, but I don't recall seeing any vintage rifle matches.

My guess is that you saw the effects of storing ammo in a hot attic for several years.