If your Featherweight bed seems loose and floppy then it might just need to have the cupped washer turned around (see Featherweight Hint on the blog here). However, if your washer is missing, then these are sold as a set of two. Use one for each side of your bed extension. (Screw is not included.)
These washers are only necessary for Featherweights that were originally black or tan. White Featherweight bed extensions are held on with pins and do not require a screw.