I must say, I had previously read that the puppets in The Wizard of Id didn't have a Muppety look (though The Spook puppet does look like he could fit in with the Muppets), so I didn't know what to expect. And just recently I was trying to imagine how the puppets might have looked. They sort of look like store-bought puppets.
The Spook was later used in the Sesame Street Pitch Reel, and something about the puppet looks a bit different to me. I think it's something about the nose... I wonder if maybe they changed the nose for a background appearance.
Aside from The Spook being in the Sesame Street pitch reel, I wonder what the chances would have been of these characters being used in more Henson productions, if it weren't for the fact that they were based on copyrighted characters. I wonder if that's why those puppets don't appear at Henson exhibets. It is good that they were able to include the pilot on Henson's YouTube channel (I wonder if they needed permission from the copyright holder. I don't think I've seen any Wubbulous World of Dr. Seuss clips on Henson's YouTube channel, after all).
It is interesting that this was produced in 1969, when Frank Oz was starting to do voices and Jerry Juhl was starting to end his career as a performer, yet Oz does no voices while Juhl does. I would have expected The Spook to have been an Oz character (it wouldn't surprise me if Oz did the physical puppetry of the character, at least in scenes where he appeared with The Wizard).
As a side note, it's interesting how in the late-1960s, Jim Henson had to let Jerry Nelson go because he didn't have enough work for everybody, giving him the occasional performing opportunity when he could, until 1970. I wonder if Henson considered hiring Nelson for this or Sesame Street. Judging by the pilot it seems the series wouldn't have had many characters (how many characters did the comic strip have at the time?), but I'm surprised Henson didn't try to get Nelson to perform on Sesame Street when the series began (especially after needing to hire somebody to perform Big Bird).