Clearly the weapon doesn't have an "Altered or Erased" serial number provided by a federal licensed manufacturer under so the wrong charges have been brought.
609.667 FIREARMS; REMOVAL OR ALTERATION OF SERIAL NUMBER.
Whoever commits any of the following acts may be sentenced to imprisonment for not more than five years or to payment of a fine of not more than $10,000, or both:
(1) obliterates, removes, changes, or alters the serial number or other identification of a firearm;
(2) receives or possesses a firearm, the serial number or other identification of which has been obliterated, removed, changed, or altered; or
(3) receives or possesses a firearm that is not identified by a serial number.
As used in this section, "serial number or other identification" means the serial number and other information required under United States Code, title 26, section 5842, for the identification of firearms
Where no law exists, I am sure Minnesota's legislature will fix that. This case will be their call to correct that. This guy's situation is one of being a political pawn. Ultimately thrown out but the State will have a new law on the books as the result of it all.