Maybe a TUNNEL RAT?
When punching in a google picture search of "m1 carbines in vietnam" this same picture pops up. The next picture shows a Plainfield.
Have seen many pictures of 'Modified' carbine stocks. Many in the Philippines.
Side note, Had a interesting talk with a guy at the range about special ammo left for the NVA courtesy of the US. I'll post about it in the next couple days after getting my ducks in a row.
Cheers;
Charlie-Painter777
pictures of m1 carbine in vietnam - Google Search