Mercenary Force

Mercenary Force
Also known as: Tenjin Kaisen (JP) Developer: Lenar Publishers: Meldac (US/JP), Nintendo (EU) Platform: Game Boy Released in JP: April 27, 1990 Released in US: October 1990 Released in EU: 1990 This game has a hidden level select.

Extra Money/Level Select

At the title screen, press Up + A + B + Select and then Start. If done right, the player will start with 50,000 yen, and can select the starting level by pressing Right at the round screen.

(Source: GameFAQs (Colin))