Well this is what Wikipedia has on the history of the term "616-Universe".
The term was first used in "Rough Justice," a story credited to both
Alan Moore and Alan Davis published in July 1983 by Marvel UK in the
anthology comic The Daredevils (and was later reprinted in the Captain
Britain trade paperback). Saturnyne uses the term to differentiate
Brian Braddock, the Captain Britain of the regular Marvel Comics
universe, from the other members of the Captain Britain Corps, each of
which inhabit different universes. The designation was later used by
the American branch of Marvel Comics in the Excalibur title, which
frequently referenced Captain Britain's early UK-published adventures.
This comic was written by Chris Claremont, who had created Captain
Britain, and by Alan Davis, the artist on the UK-published series.
Alan Moore is usually credited with creating the term. However, Alan
Davis has stated that the designation of Earth 616 was actually first
made by Dave Thorpe, the previous writer of the UK-published Captain
In addition, a difference of opinion exists regarding the selection of
the number 616. In 2005, Alan Moore's son-in-law John Reppion (who is
married to Alan Moore's daughter Leah Moore), stated on an Internet
message board that the number 616 was arbitrarily chosen by Moore and
had no significant meaning, saying it "was just a random number of no
significance chosen because people always seemed to be talking about
'earth 2' or 'earth 4' but never any higher numbers."
However, Alan Davis has said that it comes from 616, a variation on
the Number of the Beast, picked because Dave Thorpe "wasn't a fan of
the modern superhero genre" and expressed this in his stories, "such
as recording his opinion of the Marvel Universe with the designation