Here is the wiki article on the Star Tropics letter.
Originally posted by: Wikipedia
A unique aspect of StarTropics was a saga involving a piece of paper, resembling parchment, that came packaged with the game. Written on it was a letter from Dr. Jones addressed to Mike, asking him to visit him at his laboratory on C-Island. It was as if the actual player had intercepted the message and was being invited indirectly to play the game. In later parts of the game, Mike receives an enigmatic message from his uncle through a third party:
"Evil aliens from a distant planet...." "Tell Mike to dip my letter in water...."
Even for a player who owned an original copy and thus, was more likely to have the letter, it was unusual for an NES game to refer to a physical object that would otherwise just be a novelty (although Infocom games had been doing this for some time, with the "Feelies" that they included with their games).[citation needed] This prompted the player to think that Dr. Jones might be referring to an object within the game. To add to the confusion of the puzzle, putting this paper under water might damage it. Regardless, the correct course of action was to dip the physical piece of paper in water. It revealed a secret message from Dr. Jones and the number "747" that must be used in the game in order to advance.[4]
Since many rental stores and used video game retailers often do not have game boxes or manuals, it is difficult to find a copy of StarTropics with the original letter, requiring the use of game manuals or walkthroughs to complete this part of the game. Game magazine Nintendo Power was asked this question often enough that they published the "747" code as part of their "Counselor's Corner" soon after their article of the game. After completing the game a slideshow of events from the game is shown, including the letter being dipped in water and the number "747" visible on it.
For the Virtual Console release, the letter is included in the digital manual (Operations Guide) with an image of a letter and a bucket of water at the bottom. When the player clicks on one of the images, the letter dips into the bucket and the code is revealed. In the European release, the letter simply has a "click here" link at the bottom of the letter.[5]
