Brad:
Serial number doesn't make sense. I enlisted in '67, my service number begins 188xxxxx - as opposed to the one you've listed - 520xxxxx. Not only that, but even though the service numbers were issued sequentially - the prefixes were: RA (voluntary enlistment), US (drafted), NG (National Guard). If this guy was a M/Sgt, he would not have a US prefix to the service number. Don't know when it started, but in my unit, if you wanted to bring a trophy home (legally) - you had to have the unit armorer plug/weld the barrel, cut the firing pin and weld the FP hole. The weapon was then examined at base PMO and they issued the paperwork that you had to carry back to the states with the weapon.
"No man's life, liberty, or property is safe, while Congress is in session." Mark Twain