Sonic Spinball (Game Gear)
This game has a prototype article
The prototypes as found in the Sonic Retro site.
Play sound effects 0, 2, 1, 5, 6, 6 in the sound test, then pause during gameplay and press Down.
In the Options menu, play sound effects 0, 9, 0, 1, 6, 8 in the Sound Test (the screen will shake if you did it right). Start the game, then pause and press 2.
1994-05-30 P.M. FINAL REV. 20 (K)
A build date can be found in the ROM at 0x4C61. You can get it to display in the Options menu by playing sound effects 0, 5, 0, 3, 6, 0 in the Sound Test.