GEN1 Legit Mew From ToysRus Distribution
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
I've recently purchased legit U.S. Pokemon Red and Yellow Game Boy cartridges with their original batteries still holding saves.
The Red save has what appears to be a YOSHIRA Surfing Pikachu in the party, while the Yellow save has what appears to be LINKE Mew in the party (though based on the levels of Pikachu and Mew, neither seem untouched). Seller of the Yellow cart says it was from a Pokemon Stadium Tour back in the day, which seemingly jives with the LINKW Mews info here as well as so
