SFL-1 Signature: how do you know it's a signature?

Aside from the face plate that says SFL-1 Signature, how does one verify if it is in fact the Signature model? I just bought one and upon opening the inside, it has the same tube compliment as the regular SFL-1: 12At7a I know that the Signature has a different tube complement - would a 12AT7a work as well? Would the back panel say "Signature" on it? HELP!!!!