This gift was distributed in Toys "R" Us stores across the United States, distributed to commemorate the release of M08: Lucario and the Mystery of Mew.
It has been determined that 430 MYSTRY MEWs existed, though only 426 was used, and 4 were not available.
This extra 4 was created due to how the distribution procedure (when transferring to the save used by TRU) has to create 5 Mew at a time.
However, as the save maxed out on capacity, 4 MEWs, and 1 starter, was traded away.
This page holds the 4 MYSTRY MEW that were unavailable to players.
Additionally, besides the standard in-game trade Farfetch'd, every Pokémon found in the Hall of Fame of the MYSTRY Mew save has been dutifully recreated and added to here.
Recreation is based on PID, which can be used to infer the Pokémon's Nature, IVs, and even encounter slot.
Based on the data, besides Blastoise, based on encounter slot data derived from their PID,
it appears that the level shown in the Hall of Fame entries is the level they were caught at, meaning they weren't even leveled up.
Their likely use, was to be placeholder fodder, to take the brunt of attacks, while the player revives their Blastoise.
Read more here about the 430 Mew here: