I think the first two or three years of the NHRA Supernationals race at Ontario, CA where for Pro racers only. I was told that the attendance was not up to what NHRA wanted so in either 1973 or 1974 they move the World finals for sportmans and pro racers from Texas to Ontario, CA and combined both races from then on. I think the first year I help another couple race their A/SA car there was in 1974. NHRA allowed a 16 car field max in all the sportmans and pro classes and we didn't qualify
All the division champs for each catergory where seeded and the present year world champ was seeded also so that left 8 positions open for the other racers invited for all seven divisions. You had to be in the top ten in your division to be invited to qualify and the fastest eight cars under the then existing world class record your car was in where qualified
My how things have change since then