Knife Edge: Nose Gunner 

Level select 
Select "Story" or "Team" mode:
then hold C-Left + C-Right + C-Up + C-Down + R and press Right, Up, Left, Down when the level number is displayed.

Extra difficulty level 
At the Kemco logo:
hold L + R + C-Up and press C-Right, C-Left, B. Then, a "Super Hard" difficulty setting will be available.