It's dealing with the old law, the National Firearms Act of 1934, at the time the law was wrote, importers again still stayed a 1/2 inch above. Some don't have it, due to the law suit after importation started.
Under the original Act, NFA weapons were machine guns, short-barreled rifles (SBR), short-barreled shotguns (SBS), any other weapons (AOW or concealable weapons other than pistols or revolvers), and silencers for any type of NFA or non-NFA. Minimum barrel length was soon amended to 16 inches for rimfire rifles and by 1960 had been amended to 16 inches for centerfire rifles as well.
This is my understanding of them.