OK...last night/this morning's episode was an episode from 1971 (with the short-lived characters Tom, Rafael, and Molly the mail lady), sponsored by the letters B and M, and the number 4. The plot was that a rainstorm came to Sesame Street, and there were various street segments related to that. Here are some of the skits (non-street) I remember:
Mahna Mahna
Cartoon about a man inviting an M over for dinner (the M eats everything, and the man says he'll never invite an M over for dinner again...)
Cartoon about big and small (with various people and creatures saying their big, small, bigger, smaller, etc...)
Cartoon about a boy who knocked various B-objects off of a brick wall with a baseball bat (narrated by Casey Kasem, or at least someone who sounded like him)
The "B is for bicycle, B is for bear" cartoon segment (bear rides a bicycle, then goes over a bump, and hits a tree filled with bees, who chase him away)
Kermit teaching about the letter B (as Cookie Monster eats pieces off, turning it into different letters)
Kermit teaching counting with four eggs, which hatch into four chicks
An addition/subtraction short, featuring insects and lizards
Anything Muppet segment with two kids playing hide-and-seek; the one seeking has to recite the entire alphabet, then find the one hiding, but the one seeking can't remember what comes after certain letters, so the one hiding has to keep coming up to him and reminding him...when he reminds him that Z comes after Y, the one seeking says "Z", and then wins, because he found the one hiding!
Grover tried to surprise Ernie, who is actually Herry (with a BLUE nose) wearing Ernie's shirt, with a wig that looks like Ernie's hair
Ernie and Bert segment where Ernie has a piece of string tied to one finger, reminding him he has a piece of string tied to the next finger, etc., etc., until the last finger with a piece of string tied to it reminds him that they're out of string
Ernie and Bert segment, with Ernie tearing the apartment apart looking for a note for Bert, which reads: "Dear Bert, It's your turn to clean the apartment. Love, Ernie."
Anything Muppet segment with a Muppet trying to buy a hat; all the hats fit the store owner, who has a larger head, but not the person shopping there, who has a small head
The Mystery of the Four Dragons
And that's all I can remember...