Just looked it up (I'm an academian so the temptation hit me as soon as I posted a few minutes ago). Iowa started playing 6-on-6 girls basketball in 1919-20. It was the only game in the state for the girls until 1984-85, when they added a 5-player class. The 1992-93 season was the last for 6-on-6. The following season (93-94), they split into 4 classes for 5-player ball.

That would be pretty good evidence that the original purpose for the division line was for 6-on-6 basketball, because it predates the 1933 rule change creating the division line for the purpose of the 10-second count.