The S-hook is simply a 2" "8 hook" with one end bent 90 degrees then attached to the gun track under the rear gun disk. The S-hook is attached to the bottom of the thrower through the gun track by a fender washer and a allen head bolt. It is used to attach the thrower to a belt fob on the uniform as an alternative to re-holstering the thrower on the pack.
Sony Lobby GB2 thrower photos:
Fan made example: