Alfred Chicken (Amiga)
There's a dump of the CES demo for this. It does have at least some graphical differences.
Alfred Chicken is your standard issue zany animal platformer for the Amiga. Other versions were released on other formats, although they mostly had the same level designs except for the Super Nintendo one.
At the title screen, typing HELPMARK switches to a level select screen, with stages selected by pressing keys 1—B. There is a Mark Knight in the credits, one of the game's composers, for whom this code may be named.