The Medela Pump In Style Advanced AC Adapter (Transformer) is a 12 Volt Replacement AC Adapter (power cord) or a 9 Volt Replacment AC Adapter (power cord).
The 12 Volt (#9207041) is for use with Pump In Style Advanced models manufactured between 2003-2007.
The 9 Volt (#9207010) is for the Pump In Style Original and Traveler models.
If your Advanced pump was purchased in 2008 or 2009, check where you plug in the adapter to make sure if you need a 9 Volt or 12 Volt adapter. More than likely, you will need the 9 Volt.