Whirlpool Casement A/C

[appleimac]

Pulling my Whirlpool casement air conditioner out of my second story condo bedroom window, a sliding window - installed this ten years ago, and did a neat job by upgrading the cheap "filler" paper panel with heavy duty plexiglass, which I then taped GILA window film to so it matched the surrounding window.  It stopped draining from the drainage hole in the back, and was spraying water on the outside of the wall at times, plus a moldy smell at times.  Bought a/c coil cleaner at Home Depot, and a fin straightener, and a cart to transport the heavy a/c from the window to the tub - cleaned up pretty well, but what is the little door with tabs, only accessible once the unit has been taken apart - about an inch square or so, apparently behind the push-pull mechanism that would bring in fresh air?  For the life of me could not get this stupid little thing out to clean it, just a vent for the fresh air when in that mode?