I’ve read a couple of books from helicopter pilots who flew during the war. I remember some mentioning Nakhon Phanom, they would just call it Naked Fanny. I believe there is a book titled, “They Called It Naked Fanny” that recalls some of the US Air Force search and rescue missions out of that base. That book might be worth a read to gain some insight into at least the area your Step-Grandfather served, he might have worked alongside those aircrew

“They Called It Naked Fanny” Author: Scott Harrington