It would appear then that St. Cyprian is the earliest to teach BoB for catechumens. That means I don't have much greater antiquity than St. Augustine on my side, as I'd thought. But just because something isn't universally accepted or understood by 300 AD or even 1500 AD doesn't mean it doesn't have a link to the Apostles. I refer again to the Immaculate Conception.
Remember, though, that with St. Cyprian, in the EXACT same treatise in which he brings up BoB for catechumens, he introduces another grave error regarding Baptism, that there can be no valid Baptism administered by heretics. And I'm guessing that the other five Fathers who held BoB for catechumens afterwards may have just been following him. You can find just as many Fathers I'm sure who followed / agreed with St. Cyprian that heretics didn't have valid Baptism.
Is it POSSIBLE that this came from the Apostles? Sure. But do we have any proof that it did? No, we don't.
One clue is that the Fathers who support BoB clearly misapply Our Lord's reference to His Passion as a "baptism", wrongly adducing this as proof for BoB. Our Lord was baptized already, and Our Lord needed no baptism. His baptism was an extremely unique case, since by His Passion He washed away the sins of the whole world. This passage in no way proves BoB.