The coupler plays the all important roll of transferring the movement from the pulley to the rotors. The nose or pulley end of the supercharger is basically a shaft, with a flat round part that has three pins sticking out. Here is a pic of the nosedrive on a series 2. It is basically the same as the series one.
And those three pins inside the nosedrive.
The rotor gear that is driven by the coupler has the same three pins. Here is the coupler shown on the pins of the SC rotor gear