Re: Capture papers
I don't know about those numbers, presumably military regulations. But "capture papers" have nothing to do with customs. They are documents, issued by a local commander, usually at company level, allowing US military personnel to retain and bring back to the US captured enemy weapons as long as they were legal to own in the US. Since the NFA was passed in 1934, machineguns were not legal, so they could not (legally) be brought back.
But pistols and rifles were OK, and many were brought back with and without the "capture papers". The authorization covered WWII and some of Korea, but later the rules changed and today it is nearly impossible for GI's to bring back any captured enemy weapon from the middle east, although some antique and C&R guns can be purchased and brought back legally. Those have to go through customs like anything else any traveller buys overseas. Guns purchased by military personnel from PX/BX's and gun clubs overseas can be brought back; check military regulations.
A word on WWII "bringbacks." Only captured enemy weapons could be brought back legally. Bringing back a US weapon (M1 carbine or M1911A1 pistol) was illegal and was theft from the government, pure and simple (yes, I know all about how your grandfather brought back 25,000 M1 rifles in his duffle bag, but he was violating the law). It was also illegal to bring back Allied weapons, but that rule was pretty much ignored in the case of Allied weapons captured from the enemy.