I *think* this is roughly the chronology…:
Gen 1 tends to be the 15” drivers - the original Compact, Vintage, Big One etc.
Gen 2 is where the 12” drivers appeared such as this cab (Super Twelve/Super Twelve T), Big Baby, Big Twin.
Gen 3 is where they introduced their own proprietary drivers - Super Compact, Super Midget, Super Twin, Big Baby 2, Big Twin 2 and the 10” cabs
ETA: the whole “Gen” thing only really became a naming thing when the Gen 3 stuff appeared. Bit like Red Dwarf; seasons 1 and 2 were just called Red Dwarf. It only started getting the number from season 3 🤓