I wish I knew! I only remember those two, though there were probably a couple of others.(The I.C. intro wasn’t used more than the first few times the song was played, as I recall, so though I listened to the radio a lot, I may have missed a few.) But as I said, very few songs got that intro. Likelihood to reach #1 was surely part of it, but it was obviously not the only criteria, as plenty of #1 hits didn’t get the designation.
The Instant Classic thing didn’t last long. I don’t recall hearing it in 1971, when “Let’s Get It On” was released, and I know it was over before 1972 and “Papa Was a Rolling Stone.”
Maybe some other Chicago Doper will remember better than I.