A post from my old Marvel blog in which Alan Davis writes in concerning the origins of the term Earth-616 used to describe the Marvel Universe.
Alan Davis sent me the following information concerning the origins of the 616 designation for the Marvel Universe:
616 is an alternate, and some say older, ‘sign of the beast’, 666. It has some basis in Greek numerology (ZAZ?) but I don’t recall the details. Although credited to Alan Moore, like much of the other Captain Britain folklore it was in place before he joined the book. (In actual fact the only significant addition Alan and I co-created was the Fury.) Jim Jaspers, the Crazy Gang, Saturnyne, the Omniversal organisation (Avante Guarde) Captain UK, etc were all conceived by Paul Neary or Dave Thorpe. Most of the more classic elements were Paul’s (he had worked on an Arthurian Black Knight strip for Marvel UK some years earlier that utilized Captain Britain and introduced Jackdaw). Dave Thorpe, who wasn’t a fan of the modern superhero genre, was responsible for most of the more madcap or satirical elements–such as recording his opinion of the Marvel Universe with the designation 616.
The irony is that the Multiverse was created out of Marvel UK’s desire to use Marvel US characters–which wouldn’t have been tolerated at the time. Setting the stories on an alternate Earth offered the opportunity to use any of the Marvel stars without concern for continuity–but Dave had no love for Marvel characters and Alan was offered SWAMP THING before he had finished his first arc so we never took advantage of the possibility.