Keep in mind its next to impossible to have a mew on that generation of pokemon games (it was an extremly limited event) the mew is most likely hacked (although there is a very very very obscure glitch in red/blue that can get you mew without hacking) still 350 hours is quite a long time to just erase.

it reminds me of a copy of saga 2 for gb (final fantasy legend 2) it had 3 save files one about halfway through the game, one rather early, the third had just an incredible party on it, all stats were maxed out and all items were all the insanely best items you could get in the game (parasuit, Glass sword, and 7 sword) everything was aparently legit, I coudln't bring myself to ever delete it, till one day I was just playing through the game and stupidly saved over the god like save! sad thing is I have a device to back up gb saves, but I never used it. (still kicing myself to this day as I could have backed it up and been able to mess with it)

i've noticed most japanese rpgs i pick up always seem to have that godlike save on it.