For those not in the know, they were a boxed yellow popcorn snack with a sugary glaze. An unverified sentence in their Wikipedia entry claims they were the first food item to be packaged in black, which seems weirdly unlikely, but whatever. (This design page backs that up, with pictures.)
In my head, I mix them up with Fiddle Faddle, Crunch 'n' Munch and Poppycock. They all seem to have been made by the same snack company, Lincoln Snacks, which later was acquired by ConAgra., who discontinued SYZ in 2007.
I gotta admit, I think of Screaming Yellow Zonkers as the ultimate stoner munchies. Maybe it's the name, maybe the Sixties-era psychedelic packaging, maybe it's because it's too easy to eat an entire box.
Did you ever eat this stuff, or did you prefer Fiddle Faddle, Poppycock, or plain old Cracker Jack? Did you realize it had been discontinued?