This sounds like an awesome episode, Gordon planning to visit Snuffy's cave with Big Bird - but unfortunately, Big Bird has to change plans and has trouble describing the way to Snuffy's cave.
https://muppet.fandom.com/wiki/Episode_2054
Interesting line" "Things were better when I didn't believe he was real". I guess he regretted his belief.
I wonder how many episodes from season 16 have them discussing the fact that Maria, Gordon, and/or Linda believe in Snuffy, besides episode 1966. I have seen one other guide that mentions Maria believing in Snuffy, otherwise it doesn't seem to come up much that season (unless the guides just omit such details, which can be understandable in some cases). I wonder how many people missed the season premier and then got confused when they saw later episodes that season where they mention one of them believing in Snuffy.
Another interesting thing is that Big Bird needs an adult to sign a note granting permission for Big Bird to spend the night in Snuffy's cave. I want to say that there were earlier episodes where Big Bird spent the night there, from before any of the adults' believed in Snuffy, did he need a written permission slip then (of course I can't fully remember if he spent the night before then)?
Recently I thought of something regarding the truly lost episodes: Since a lot of lost season two street scenes were included in the early years of Sesamstrasse, I wonder if Sesame Workshop sent their only copies of the episodes to be dubbed, and then some simply didn't get sent back. Could the original English language copies be in storage with Sesamestrasse episodes?