I wonder if that G3 wasn't a Pak or Iranian one. If nothing else, Soviet SOF had a huge mission for operations behind NATO lines in the Brandenburger mold from WWII, and actually trained for it reasonably intensely. There were always rumors of GRU or KGB female members in the Women's Peace Camps set up outside GLCM bases, for example. True or old wives tale to keep the Sky Cops interested in their jobs, who knows. Plausible for sure.

NORK SOF had tons of M-16s as well, and since their first day of the war rear area disruption operations were on the Soviet mold, I wouldn't be surprised if those rifles were also not Vietnam origin, or even purchased off third parties in grey or black markets, like the infamous 80 MD500 helos the North Koreans bought.