This picture of John Lennon and son Julian on the Happy Days set during his "Lost Weekend" phase surfaced a few years ago, and, yes, it is genuine!
If you don't believe me, take it from Anson Williams!
Lennon's then-girlfriend May Pang posted the pic on her Facebook page. She and the Lennons visited Paramount Studios in 1974. I like to imagine John asking Fonzie to make the jukebox play some Chuck Berry, then being heartbroken at learning he can't actually get a burger and fries from Arnold's.
It's a shame John wasn't around years later to contribute material to the "American Musical" episode we discussed in season 1.