Famous, they said.

It’s a peculiar concept for a cover; show the MM in human incarnation, holding a drawing of one of the puppet sidekicks, then show him below, regarding the hole in the fabric of space that has allowed people from the drawn world observe the goings-on of those encumbered with flesh. He alsoh as the puppet sidekick except he’s wearing a smock.

I’ll bet he’s supposed to be a scarecrow. When they drew scarecrows they always had that jagged thing around their necks. And you know the guy on the right is some sort of Explorer Professor - perhaps Prof. Explorer - who travels the world looking for a cure for his freakish hands. As for the Merry Mailman:

He appeared in the musicals Anniversary Waltz, The Desert Song and Babes in Arms, where he played the role of "Valentine" the character about whom the almost-instant classic "My Funny Valentine" was sung -- long before Frank Sinatra's hit recording revived it in the 1960s.

That's a song about a guy? You can find more about the show and the Mailman here. HIs name was Ray Heatherton.

Yes, indeed. Joey’s dad.