Judge Goodwin, a President Clinton nominee, noted that "serial numbers were not broadly required for all firearms manufactured and imported in the United States until the passage of the Gun Control Act of 1968," and the ban on possessing a firearm with a removed serial number didn't come about until 1990.
Given that, he found the law barring the removal of serial numbers fails the test established by June's Supreme Court case, New York Rifle & Pistol Association v Bruen. Specifically, to justify a firearm regulation, the Supreme Court said "the government must demonstrate that the regulation is consistent with this Nation’s historical tradition of firearm regulation."
Though the Bruen test is only a few months old, it's already animated several rulings against gun laws,