William "Rosko" Mercer

For a while now, I've wondered about a narrator featured in some old Sesame Street segments. Here he is doing the Queen of Six:

And he's at the beginning of the King of Eight:

And he's the voice of "Limbo" in this Scanimate film:

Thanks to a random commenter on YouTube, he's been identified! It's William "Rosko" Mercer, a prominent New York DJ. Here's more of that unmistakable voice:

All the above spots were done by Jim Henson, so Rosko must have been a favorite of his. Rosko's NYTimes obituary is here.