Must have been even earlier than that. I have always said "because of your just punishments" and my first Confession was in 1955.
Of course, being taught in the Baltimore Catechism method, I understood exactly what that meant. That would be Hell. Which, if God gave us what was just, would be the final end for everyone. It's only through His mercy that we have any hope of Heaven.
But as you correctly state, that DOES mean Hell.
That is how I learned the Act of Contrition as well and this was in the 70's.
I understand many of the authors points and agree with them but I slightly disagree regarding the Act of Contrition.
Aren't Catholics supposed to be striving to confess our sins out of a love for God as opposed to a fear of Hell (Perfect Act of Contrition vs. Imperfect Act of Contrition)? Fear of Hell is a good start but it should not end there.