Does anyone know why Dana Elcar had left the show? He's my favorite sheriff.
Unfortunately I've never seen a definitive answer to that. His last appearance (in #329) taped on Sept. 15, 1967, and when the weekend was over, Vince O'Brien had taken over his part (in #328, which was shot out of order on Sept. 18). Elcar was on the show just over a year, so perhaps it was a contract issue? [Note: The Aftra strike hadn't started yet, so if Elcar was given his walking papers, it wasn't because he refused to cross the line.]
About a month later, Angus Cairns had the role briefly. Then O'Brien returned, but we got Alfred Sandor at this point, and O'Brien was the series' final George Patterson. I think I got that right. Anyway, it's all pretty schizo.
Of course, the part was recast again for HoDS, giving Dennis Patrick a role in the movie.