Not to quibble, but you could say the commas make the part about the magazine an independent clause. In other words, that the writer isn't implying the mag is one of the parts "that would make the weapon fully automatic." Granted, I'm probably giving the writer more credit than I should.
I think there's an outside chance that this dude read the Sten thread here and ordered a parts kit. And I'm not even joking, much. Because that article could easily be describing a Sten kit. To the media (and probably the cops), "the parts" are the FA bolt and the selector button.
Amazing, the world we live in. From reading the article, I bet the only "unlawful" things they found were the fentanyl... and the mag.