It's an amazing coincidence that last week, the pages for episodes 1360 and 1814 were moved to sandbox pages due to the scripts not being the final aired versions (are there any more scripts that are not "final aired version" or at least hard to determine?), then just a week later they were able to obtain video of both episodes (with 1814 being more of a surprise, for obvious reasons).
Interesting, according to the old edit of lost episodes of Sesame Street, that the script provided did not contain Monsterpiece Theater: The Sound of Music, at least one later script cited this one as the first appearance while the script for its second appearance (I think 1828) cited itself as the first appearance, and then it turns out this episode was in fact its first appearance. I think it aired a few more times between seasons 14 and 16, I wonder if most of them list 1814 as the first appearance.