That's a fascinating observation. One would think that a particular song was likely "the first" and the format was seen as "good" then emulated. I'd love to have it pinpointed. Well, I see you say there were "very few" before that point, indicating that there were "some". Perhaps some "record executive" even played a hand in it, telling artists "this is what I want more of". It will definitely be interesting to see if anyone has any definitive insights, but in and of itself it's a fascinating thing to contemplate.

What kinds of formats have you noticed before the AABAs came into prominence? I tend to listen to a great deal of things from similar eras, or closer to them than modern, at least. This moment I have Lead Belly in my ears, but I've never paid much attention to such structural aspects.