The year they started calling it "The NFL", 1947?

[oldecapecod11]

Contrary to the belief of some, it is more probable the "boros" and the like were first used by tabloids -
not papers that published in broadsheet form.The Chicago Tribune has always been a broadsheet
but added a tabloid format within the last ten years-or-so.
By using a few self-bastardized words, a tabloid can create a tad more headline or lead space thus enabling
the use of larger type which might be more eye-catching - especially on a newsstand
(for those who remember newsstands.)

But, this is for other matters, I.E. the probable use of NFL as an acronym.

If one were a betting man, it might be safe to place a wager on 1933 - the year of the first Championship
played by Division winners.
If one were to search, the suggested issues would be those just prior to the game and those immediately
following.
Certainly, you will see NFL Champion or NFL Championship among those pages -
probably a lot more than once.

If one were to be forced to pick one particular paper, the smart bet would be The New York Daily News, a tabloid
that gave a lot of attention to The Giants of New York - one of the teams that just happened to be involved in the 1933 NFL Championship Game.