However, there was also a Gardner Fox on Earth-C, the Captain Carrot universe (which was later reclassified as an alternate dimension, not an alternate universe).
I forget — when was Earth-Prime discovered in the DC Universe? I have vague memories of some of DC’s stable of writers helping out the Flash (and thus Earth-One) back in the 70s using some kind of treadmill-like device to bridge the universes. Was Carmine Infantino revealed to be the villian?