Not long ago, I got a copy of a master reel of Sesame inserts (primarily season 19), complete with slates. I'm uploading them here for the curious, but since they have taping dates on them, it can better help us ID first appearances of certain bits.
For a number of years, I've thought about how it seems like Jim Henson and Frank Oz didn't perform together much in seasons 19-21 (and I could be wrong about that), as most segments I know of with either performer that I've seen an official source for being from one of those seasons only has one of them, and there seemed to be quite a few segments those years with just Ernie that were about him being without Bert (though I haven't seen any official sources for the dates of those segments), but then I looked at some of these and saw that one segment with Jim Henson was recorded on September 10, 1987 and then a few segments with Frank Oz were taped the next day, so they were on the set one day apart from each other (though the Jim Henson sketch in question is a Monsterpiece Theater sketch, so Frank Oz was also in it, though it's possible Kermit's parts and Cookie Monster's parts were shot on separate dates).
Well, the Tree Report and "Opposite Stuff" slates are the same date, so that must've been a Jim-centric day. Also, the Waiter's Memory, Tuba or Not Tuba, and the majority of Muppet/Kid Moments were filmed a day after Gone with the Wind, so that must've mostly been Oz/Nelson/Hunt. (And jeez, three different sets all in one day...)