WW2 US Army Air Force shoe insert flying electric

Vintage WW2 US Army Air Force shoe insert flying electric Type Q-1. Says inside on tag size mediun 7 1/2 to 8 1/2 shoe size. Made by General Electric they are used by the pilots to keep their feet warm when flying. They plug in and have heated coils inside. They are in good condition for their age, there are a few small holes inside.