The NES version of the game came with a letter inside the manual from the in-game character Dr. Jones. In the game, Dr. Jones tells you to use the letter, however many players assumed he was referring to an item in-game, leading to mass confusion. This lead to Nintendo Power publishing the code on the paper (747) in their "Counselor's Corner" section. In the credits, where scenes of the game are shown, the wet letter is among them.
In the Virtual Console release, a digital version of the letter was included in the Operations Guide. In the Nintendo Switch Online version of the game, the code simply isn't included.