As the VC was present all through the South acting undercover, I can see them buying it locally. Far better to be questioned by the authorities about a local gun than found with a Chinese issue pistol. All is speculation because they can't talk.

Strange things happen in war. I foolishly did not buy a Winchester Model 670 with US M84 scope that had legit capture papers. The US built up these rifles on the cheap and gave them as "sniper rifles" to local tribal chiefs.
Appartenlty the VC had captured this one and some GI recaptured it and got it home with papers.