This photo was posted on Facebook via The REAL Peter Tork:

Explanation:

“Happy Days had just been on the air a short time in 1974 when the cast noticed a familiar looking man hanging around in the studio one day. It was none other than former Beatle John Lennon, along with his then 10-year-old son Julian. ... Lennon stuck around for the entire day of shooting, drawing doodles for the crew and hanging out until they wrapped the shoot after some 12 hours...”

From the looks of it, this was from Lennon's “Rock n' Roll” period. Was he still on the outs with Yoko then? Hanging out with May Pang?

Yes. Just did the Google search. According to BuzzFeed, it was taken in January 1974.

So this was kind of a low point for him (artistically, personally) and a high point for “Happy Days.” The early shows were quite good, and the title almost ironic. Things invariably went badly for Richie, but he had family to fall back on. It was only when the show shifted from Richie-centric to Fonz-centric that it lost its sense of irony and became a huge hit. 

But that photo's nuts. Isn't it? It's like some odd amalgamation of my childhood/teen years. It would be like a photo of Evel Knievel hanging out with the Twins at Met Stadium. Or Thor Heyerdahl and Lindsay Wagner visiting “The Planet of the Apes” set. Worlds colliding.