One thing I'm confused about, and this might be more suitable as it's own thread, is when Martin Robinson joined the show. Most sources I've seen (the 40th anniversary book, Robinson's official bio, etc.) say he joined in 1980 (Sesame Street Unpaved says 1979), which would be season 12. But in the recent Tough Pigs interview with Michael Earl, he said that he first met Robinson on the set of The Muppets Take Manhattan, which came a few years after season 12, and also mentioned they were looking for a new performer to perform Snuffy when they needed to let Michael Earl go.
I also checked the season 12 credits on YouTube, and he's not in the season 12 credits. I wonder if maybe he got to perform on the set a day or two, perhaps when Michael Earl was off (I wonder how well the performers remember the names of performers they work with for just one day). Maybe it was a situation similar to Kevin Clash performing occasionally as early as 1980, before officially joining the show in 1984 (though the difference is that official sources never acknowledge that Clash was on the show before 1984, while they do mention Robinson first performing in 1980).
Maybe this would be something to ask Robinson on his blog (though I've tried sending an e-mail and it wouldn't let me process it).