If you are worried about theft but do not want to use the theft proof adapter, you might be able to drill and thread a small hole in the cap for a 4-40 set screw to lock the cap on the radiator. Just remember to carry the allen wrench to loosen the set screw to remove the cap. To be honest, I have not done this on a Model A but did do it for my Model T. The set screw was in the back and not noticeable. I put a small dimple in the radiator neck to set the setscrew into.