Back in the good old days (insert year here),
A Mew giveaway was arranged by Bergsala, the Nintendo distributor in Denmark,
through the official Nintendo magazine they were running at the time.
They had participants send in their cartridge, and then they put a Mew on the cartridge.
It was done in two rounds; one was a limited giveaway where approx. 30 people were selected in a contest,
and then a general distribution later on with the same method.
@suloku found the information regarding this online, and contact the player (using alias Michael, who wants to stay private),
and managed to get his cartridge to back up the save and contribute the Mew online.
While this Mew is already trained, I've attached a version which I reverted it to an "untrained" state.
Big thanks to @suloku and Michael for this historical Mew find!
THIS POKEMON WILL NOT BE LEGAL WITH ANY VIRTUAL CONSOLE RELEASE
(AND BY EXTENSION, CANNOT BE TRANSFERRED TO GEN 7 AND ONWARD).