The Mew contained in this Pokemon Blue save file is legit and was obtained from a ToysRus distribution in 1999.
The first 1,500 customers at each Toys "R" Us store around the United States received a special 'Peel & Win' sticker card on November 26, 1999. 200 at each store contained the text "Caught Me!" underneath, which allowed players to receive a Mew from December 8 to 12, 1999.
Legitimate Mews for GEN1 Pokemon games are extremely rare these days, due to the fact the save games
So, I can't find the time to make the post with the mews I wanted to make (my priority was getting the info, not getting anything new unfortunately), so I'm uploading this save with the info for now, as I've been requested it and there's no reason to keep it (I was waiting until I made the big post, but it's been over a month and still not the time for it).
The save attached to this post contains a Mew owned by Cirellio, who I found over at nintendoage forums. He worked at Toys R Us at Peoria