SCIENCE GYROLL FIN PAD/SOCK INSERTS
It's a 2mm fin sock and a fin heel pad in one! You can move them from one pair of fins to another. They are not permanently attached. The sock is attached to the fin by a Velcro pad that wraps around the heel of your fin, and also by velcro straps in the drainage holes of the fins.
Large: 10-11 US
XL: 12-13 US