This gift was distributed to players in the US via the Pokémon Pass app. By accessing the app in any Best Buy store, players get serial codes that can be used to redeem this Mewtwo.
The giveaway dates on the app is between November 4th to November 14, 2019. That being said, some players have been able to access it as early as November 1st.
The redemption of the code is till March 5th, 2020.
As events for Let's Go Pikachu & Eevee are not region locked, the serial code can be redeemed on Switches of any region.
Format Ver.1.0.8-3b4, Post Updated Date:20191102_0311
What can be done with these files
PKM creation with PKHeX (drag & drop the wondercard file in the spot where you want the Pokémon)
In-Game Mystery Gift redemption with Switch Gift Data Manager