B is belly door on alpha: (from FireballJackpot)
"Belly Door" is the door where your oval graphic is displayed. PULL on the lever on the right side of your door to make the Belly Door open towards you. (Watch it! - The Non "Shock Absorber" style will slam down on you). Interlock switch is in the upper left corner.
A1 and A2 door codes:
M (Main Door),
L (Logic Door),
S (Stacker Door),
V (Bill Validator Door),
B (Belly Door),
D (Drop Door),
C (Middle Door),
U (Upper Door),
d (Deck Access Door),
s (Spare Door),
t (Bonus Topbox Door).