I lifted this photo from Jason Hardy's MACV-SOG series of books, volume X. This is one I would like to own.

Note the folding stock AK47. Relatively uncommon compared to the full stock version in this time period. More than likely captured from the Viet Cong. It appears as though he is holding on to a forward vertical grip. If factory original, it is most likely an Eastern Bloc weapon i.e. Romanian which were known for that style. Also note the skull in his other hand!!