These can only be viewed during debugging. There also appears to be a message for when completing the game on SUPREME MASTER (originally called simply MASTER), even though the final version of the game contains no similar message and simply plays the ending sequence. Apparently the WARRIOR difficulty was originally called NOVICE. There are unused text strings in both versions of the game for what appears to be early versions of the ending messages that you'll get when you complete the game on one of the lower difficulty settings in the English version. The palette also appears to be different from the one used in the final game.
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The manual for Double Dragon III: The Sacred Stones, which features a plot summary of the second game, for some reason uses a screenshot from an early version of the game in which one of Marian's alternate designs is used. The first one from the left is the one that is actually used in the game, while the other two are unused designs. There are three different sprite designs for Marian when she appears just before the final battle in Mission 9. Bit 5 of the debug variable (to set it, use the Game Genie code AXNYVYAA) enables free walk mode, which allows you to walk to any part of a level instantly.