The Thumbport allows greater balance in holding the flute. It is comprised of a C-shaped shell and a thumb-like extension. The Thumbport helps the player to place the right thumb at an efficient spot and provides additional support.
The Thumbport II has a lower default angle for the thumb. It's a little thicker and more rigid. It touches the flute tube less. And, its extension arm is a little shorter (when attached to the flute, it may go into the case easier for some). Also the end of the extension has a more rounded shape.