Help needed identifying helicopter

My uncle Frank (my mother's younger brother) was a marine and flew helicopters in the Vietnam War. He's currently got cancerous tumors all through his brain and the prognosis is grim. He isn't always lucid and has trouble communicating.

My mother wants to try to identify the helicopter that he usually flew. This is the only photo that we have of it. It was supposedly a rare one...only 8 or so of them were made, according to my mother, but I'm not sure of the accuracy of that info. He flew many different types of missions, but she said that he mostly worked in recovery of downed copters...? Again, not sure of the accuracy of the info.

If anyone has any idea what helicopter this is, I'd really appreciate it. I know it's a long shot, but you'd be doing a favor for a mom who is grieving for her little brother. Thanks!

Edit: apparently it's a variant of a Sikorsky made for the Marines.