It is very often claimed that the scripts for Mork and Mindy would explicitly mark places where Robin Williams was to improvise, using language such as "Mork does his thing" or "Robin does something funny". Examples of such claims can be found on Slate, GQ, the Straight Dope Message Board, and even right here on the Movies & TV Stack Exchange.
On the other hand, many people say that this claim is false or exaggerated. Some people say that the scripts never specially marked gaps for ad libs. Others say that the gaps were marked only in one production script as a protest by the writers, or that the writers once produced a single fake script (which was never actually used) consisting of nothing but the instruction for Williams to ad-lib. Examples of these counterclaims can be found in an article (and the associated comments) on The Geek Twins and in the aforementioned SDMB thread.
So what's the true story? Did any Mork and Mindy production script ever explicitly mark the places where Robin Williams was supposed to improvise? If this was a one-time occurrence, which episode was it? If it happened more than once, then when did the writers start doing this, and how often did they do it?
As a lot of print and Internet sources repeat rumours about this matter uncritically, I would prefer that any answers to this question rest on evidence from the original scripts, or failing that, from published, firsthand interviews with the show's writers, actors, or other production staff.