The PI-101 serial number is the one you have to use to get your extra Pi-101 cabinets.
It is equivalent to the Two Notes Activation Code.
If you have already completed your registration, open the plugin and go to :
Menu->Add Two Notes Activation Code and enter this number.
Your license will be renewed and your new cabinets automatically downloaded.
This requires an internet connection.
If your registration is not completed yet, you will be asked to enter it during the registration process.