Back in the end of year 1999,
Toys "R" Us in the United States and Canada ran a lucky draw campaign,
via a peel sticker, that gave players a 1/5 chance of getting the Mew.
The first 1,500 winners (per country) that went for the redemption will be able to receive Mew on their copy of Pokemon Red, Blue or Yellow.
Around the same time frame, magazine Nintendo Power also did a giveaway to the first 1,000 applicants.
Various other information regarding this can be found here.
OT: YOSHIRA or YOSHIRB
TID: Random (00000 to 65535)
THIS POKEMON WILL NOT BE LEGAL WITH ANY VIRTUAL CONSOLE RELEASE
(AND BY EXTENSION, CANNOT BE TRANSFERRED TO GEN 7 AND ONWARD).