That Mystery Guest skit has one of the most unusual character selections I've ever seen: Guy Smiley, Don Music, Cookie Monster, and Sherlock Hemlock! Aside from large ensemble skits or songs, I can't think of anything else with such a diverse group of Muppets.
I feel the same way about another game show sketch, What Is It?, featuring Guy Smiley, Grover, Biff, and Prairie Dawn (and I think Gladys appears as well).
I often wonder about the Anything Muppet patterns. There's clearly multiples of a few, but how many AM characters have eventually gotten built into their own, more permanent puppet? I recall a Henson or Sesame Workshop insider saying on a Muppet Wiki talk page that that usually happens when an AM design is used enough, but I wonder the extent of it. The Count most likely has had his own puppet for years, but I also recall seeing an '80s interview with Jim Henson somewhere where he talked about The Count as if they were still just applying the right parts to the AM pattern when needed.
I'd like to think that The Count, Mumford, Biff, Sully, Guy Smiley, Mr. Johnson, Simon Soundman, Sherlock Hemlock, and some others were major enough to have been built into their own puppets at some point. There are characters who seem to look a little different from appearance to appearance, maybe they just never became major enough. I feel Little Jerry and the Monotones all look somewhat different in each appearance, and I think Mr. Johnson also looked a little different each time until maybe around 2007.
Whether or not they eventually made a more permanent Biff puppet, during the 1970s it seems like he sometimes looks a little different. Often his face placement looks a little off, sometimes I feel his nose looks a little bigger than usual. And while we rarely see him without a hat, I feel like sometimes his hair looks a little different (though in some cases I feel the hat was put on slightly differently than usual, maybe the hair was put on a little different as well).
When I first saw a still from the "We Coulda" number, which is probably the most Biff was ever seen without a hat, I was surprised by how long and messy his hair looked. Then I watched some Biff scenes, ones where he's wearing his hat, and looking at how his hair looked with his hat on there, thought it did look like a lot of the hair sticking up was showing pushed down to the sides when he wears his hat. Then I saw the whole segment, and with the four parts where he's shown without a hat, feel like it alternates between how I expected Biff to look like without his hat and not. Though I think maybe they combed his hair for the scenes where he's an actor and a writer. And in most of those shots, we only get a view of one side of his head (when he's a doctor, we see a side view, and a little 3/4 angle of his face, we get two quick shorts where he's looking at the camera... and when he's a president, we do see multiple angles of his head.... and in some of those shots, he's a little far back from the camera). And yet in that same number, when he's wearing any kind of hat, I feel like the hair doesn't seem as messy/sticky (especially not when he's wearing a chef's hat, though that's another quick one where we just see the front of his head). Even if they were still just decorating the AM pattern to be Biff, I doubt they would have just replaced his hair for multiple parts of the same segment (especially when you consider that in all those imagine spots, they still wear the majority of their regular clothes, mainly just changing headgear).