OK, I usually don't need any help, but this time I'm stumped, so I'll ask the best and brightest.

I've got a Schwinn Superior frame made in 1978. It's got Huret dropouts with integral derailleur hanger. The hanger has the notch at 4 o'clock for the Huret derailleur tab, but it also has another notch starting at about the 1 o'clock position. What is this second notch for? Not Campy. No, it's not a backyard hack, it's too neat and clean to be a hack.