By Aidan Malley

Published: 06:45 PM EST

In an aggressive response, unofficial Mac clone builder Psystar has made a controversial claim that Apple doesn't legally own the US rights to protect Mac OS X, invalidating a major component of its lawsuit.

The addition to Psystar's mounting defense was filed last week in the Northern District of California San Francisco court playing home to the legal entanglement.

In its new submission, the Florida-based PC builder argues that Apple's complaint should be tossed outright as Apple didn't use proper procedures to register the copyright for Mac OS X. Without that copyright, the Mac maker is "prohibited from bringing action" against Psystar for DMCA violation claims and other copyright-related allegations.

The amended response also reiterates Psystar's earlier concerns that Apple is using a startup check in Mac OS X Leopard to block unauthorized systems from running the software. In the earlier retort to Apple's revised lawsuit, Psystar argues that Apple isn't using copyright protection as a failed check merely crashes the system.

Whether the new claim of invalid copyright can be sustained isn't yet clear. However, initial searches for copyrights through the US Copyright Office reveal that Apple does own at least a disc and manual copyright for Mac OS X Leopard published on October 26th, 2007 -- the day the software became available to the public.

Even so, Psystar is steadily becoming known for turning to unconventional interpretations of the law to try and thwart Apple's lawsuit, which itself has gone to the extreme of suggesting that secret contributors have helped Psystar get to the level of business it has today.