If Priscilla hyphenated her last names - “Priscilla Beaulieu-Presley” then she would have been billed up front. Note that in her first few appearances, she was simply billed as “Priscilla Presley”.
Dack Rambo got into the credits because they needed him to tie up Jack’s storyline in season 10 and he wasn’t under contract to return for that season. That was apparently part of his demand for coming back and doing his 13 or whatever episodes. Good for him. He didn’t have the clout to demand opening credits bIlling when he first joined the show, and Dallas didn’t hand that out to newcomers (until the last couple of seasons). Until then, the only newcomer to appear in the opening credits during their first season was Donna Reed. And she had the clout to demand it.
