If your Featherweight Bobbin Winder seems loose and floppy then it might just need to have the cupped washer turned around. (The cupped side faces away from the head of the screw.) However, if your washer is missing, then this is a replacement to be applied to the Bobbin Winder screw.
Bobbin Winder and Screw are not included. Please retain your original screw and slip the replacement washer with the cupped side facing away from the head of the screw.