Early ones have "Telecaster" type knobs and no Blues To Solo or Clean Gain trimmers underneath; later ones have the 'ridge' knobs and the trimmers. There may be transitional ones with one or the other feature, but I can't remember which way round.
I don't know if there's any real tone difference - I've owned three, one earlier and two later, and they all sounded slightly different, but not enough that I would think there was a real change. They all varied in the amount of annoying switching delay too .