It's his cell phone Frank ! This thing is weird all around. The Huey is so brand new that it has that new car smell, the door gunner has what looks like a M1919 but none of the usual ammunition gear, the belt is just draped over his leg. No GI would wear that type of blouse in the field, his cover is just a aircrew helmet that came with his new huey. The Captain or LT for Jim, has on trousers that are not like I've seen, no leg pockets like jungle fatigues we had. They look like rain pants, his coat man's combat tropical is what we had, He is with some Viet unit and what is that he's carrying? a satchel charge, medical kit? That early it could be anything. I wouldn't be surprised that it was a Naval Surgeon or something like that out getting some combat casey photos. The Viet in the background has what looks like an ice chest lid, "where's the beer?"
My vote on the tank is a T34/85