The ASP-121 whip is not threaded. It comes with and fits into an adapter (held by a set screw) which is threaded to fit into the top of the ASP-570 spring. It all works together with the ASP=3 ball mount and is what is correct for use on a CHP car.
See the product tech sheets for all these parts here:
http://public.fotki.com/Classicchp/s...ang_technical/