Fish Dude

Fish Dude
Also known as: Uoozu (JP) Developer: ANex Publishers: Towa Chiki (JP), SOFEL (US) Platform: Game Boy Released in JP: October 5, 1990 Released in US: April 1991 This game has unused code. This game has hidden developer credits. This game has a hidden sound test. This game has a hidden level select. This game has regional differences.

Dude. It's a fish. Who is also a dude.

Hidden Text

This string is found at the beginning of the ROM at 0x70:

                
            1990
  SALIO & ANex  
                

Regional Differences

Japan	USA

Besides the different title screen that comes with a title change, the password feature was changed. In Uoozu, press Start on the title screen and on the next screen A or B to change the stage/level and then press Start to input a password. The passwords are also four characters long instead of just two in Fish Dude.

Passwords

The Japanese version of the game includes 6 hardcoded passwords. To activate them, hold Down and press Start on the password screen after entering them.

Password	Effect	Screenshot
STAF	Opens a staff roll.
MSIC	Opens a "sound test" screen where you can play each of the game's songs and sound effects. Each song also includes a title. Press Start to return to the previous screen.
FONT	Switches the font out with a slightly larger one that fills more of the 8x8 tile space. Enter the password again to switch back to the default font.
MODE	Opens a "setup" screen, allowing you to set the number of remaining lives (0~7), difficulty (0~3), and change the in-game colors to a "negative" version.
MAPS	Opens a "map select" screen.
XENA	Allows you to choose levels A, B, and C.

(Source: @yamada_arthur)

US Version Passwords

Due to the changes made to the password system, all of the above were disabled in the US version. The code is still sitting in the ROM unreferenced and can all but "XENA" can be (sort of) re-enabled via Game Genie codes. With the desired code active, simply go to the password entry screen and press Start.

Game Genie	Effect
03F‑6FF‑5DF+32F‑70F‑A2A	Staff Roll
76F‑6FF‑5DF+33F‑70F‑A2A	Sound Test
9EF‑6FF‑5DF	Font Toggle, due to the hacky access method, will appear garbled.
C9F‑6FF‑5DF+35F‑70F‑A2A	Setup
38F‑6FF‑5DF+37F‑70F‑A2A	Map Select, due to the hacky access method it won't function.

(Source: cah4e3)