Nonsense!
The laws, 2 involved, are clear. Certain receivers are banned. When "off list" receivers are part of a complete weapon, that weapon is still illegal if it has a defined combination of evil features.
Given a list of banned weapons and the evil features formula, 6th graders could make an accurate determination within a couple of minutes.
There are exception to the rule. I've witnessed more than one gun enthusiast question the legality of an unmodiified magazine release, without first determining if the weapon was center fire.
Then it appears that the two policemen are guilty of false arrest, should be prosecuted, fined, and imprisoned.