Codes in book that comes with the game

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The Super Mario Maker book (also available on Nintendo's website) has a number of pictures in it with a key icon followed by a 4-digit number.

I assume these are used in-game somewhere... but what are these numbers used for?

super-mario-maker

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(2021) BOOK OF MONSTERS CODES FREE COINS ALL NEW SECRET OP ROBLOX BOOK OF MONSTERS CODES!

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Andrea Piacquadio, Marta Wave, RODNAE Productions, Keira Burton