There are a lot of experts on TB who know the details of the versions better than I do, but I own cabs of both versions and the series II's that I own have that designation included on the front name plate. They also have everything on the back panel printed. My series one's have the model, impedence and power rating hand written on the back panel. The grille is also different, but I believe some early series II's had the same grille as the series ones.
"The truth is more important than the facts." - Frank Lloyd Wright