Don't get your panties in a twist people, it's probably nothing more then a black carted game's exterior over an OoT's board with an OoT's sticker on it.
I know there's a ton of black N64 carted games out there, and it's a ton easier to open the (likely shitty sports games) black cart up and swap out the inside and outer sticker for another game then it is to custom paint a grey cart.
That'd be my guess.
Y'all can do something like this too pretty easy with a gamebit, original copy of a game and someone selling N64 front labels.
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To-do manuals: Arabian Knights, Gulliver Boy, Dark Law, Brandish 2, Flying Hero
Finished manuals: Neugier, Wonder Project J, Burning Heroes, Star Ocean, FEDA, Monstania, Chaos Seed