I don't think they could patent it given the simplicity of most takedowns, the number of gunmakers who offer takedowns, and the gunsmiths who modify other guns into takedown configuration.

Any maker who tries to patent the typical takedown system would be trying to secure a patent on a screw and a threaded hole. If there is a lock on it to secure it in position, the locking method was likely stolen from some other industry (example, the "button lock" on some takedown lever actions).

Richard.