Here's one article about it. It's more about the legal wrangling, but Hamill is featured a bit also.
http://www.wired.com/2014/11/batman-home-video-finally/
Here's the part about his personal set. It just says video tape. Late 70s it could have gone either way; VHS or Betamax.
“In the ’60s, it was the Beatles, Batman, and Bond,” says actor Mark Hamill, a devotee who would later provide the voice of the Joker in animated and videogame Batman properties. “There was nothing else like it on television. Adam is so underrated as an actor. He walked the line between sincerity and parody and set the tone.”
Hamill used his post-Star Wars celebrity to convince Fox to make videotaped copies of all 120 episodes for him. It was the only “official” collection any fan would see for the next 35 or so years.