Anyone know how to tell what kind of ball end shower arm you have, in order to determine which adaptor to purchase to turn it into a regular threaded arm? There seem to be at least three of four different adaptor (american standard, gerber, etc.) Perhaps someone knows the diameter of the threaded piece behind the ball joint for the various makes? I'd take the shower head into a store and just compare it to the adaptors, but it seems like not many places actually keep the adaptors in stock. Any help appreciated.