The 4101 (Z101) Thumb Screw is part of the Quiver/Clamp Assembly. It is the piece that screws into the Male Clamp and holds it to the Female Base Mount. If you want to remove your Retriever from your bow, a quick way to do it is to loosen the Thumb Screw and then pull the Retriever stem down and out of the Quiver/Clamp Assembly. There is a small O-ring that holds the Thumb Screw in place. It is sold separately (4109).
WARNING – California Prop 65