Did Mork and Mindy scripts really mark the gaps for Robin Williams's ad-libs?

Did any Mork and Mindy production script ever explicitly mark the places where Robin Williams was supposed to improvise?

No, this did not happen.

During the time of Mork and Mindy being a major hit, there were rumors circulating that Robin Williams would ad-lib most of his lines.

From a series of interviews with the director and several writers of Mork and Mindy:

• Howard Storm (director)

  There were articles saying that the writers would just write, “Robin does his thing,” and then Robin would just ad-lib for 22 minutes. Which is ludicrous. There was no way that could happen. The writers were writing great stuff for him.

• David Misch (story editor)

  One of our lines when people asked about ad libs [was that] we’d say, “We’re up till four in the morning writing Robin’s ad libs”.

  He didn’t do extended ad libs, but what he did that would be so brilliant were these little things—a line here and there, a word here or there, a face, a voice—those were the things that blew people’s minds. And they may seem very tame now, but at the time, nobody was doing that on TV. Everyone just did the script as written. And he mostly did, [but] he would throw in little things here and there.

  And the other thing is, we stole from him. He stole from many people himself. And we would go see his act, and we would throw in things to the character that were things he loved doing from his act. He never objected, he loved doing things like that. So, you know, there was give and take all the way through.

  But in any case, yeah, there were really long hours and tremendous agony over storylines. And like many shows of that time and some of this time, at the end of the writing process, you would go over the script line-by-line—the entire script—with everyone. Eight, nine, ten, twelve people in the room, trying to think of funnier lines for every line in the script. So it was a tiring process. But I think it was the way pretty much all shows were run back then.

Other writer interviews also talk extensively about how Robin Williams was intimately involved in the development and dialogue of his character, and sometimes even the show's skit!

Because of this, perhaps the inclusion of [Robin does his own thing] in the script would have been counterproductive to the natural flow of the show's development process, and against how Williams was being managed as an actor.