This photo shows the air hole (angled under edge of brass disc) and the positive screw with it's insulator (red arrow). The insulator can easily get out of place if positive screw is removed and disc turns. The air hole in the deck is jut above the screw driver's tip.
There are quite a few photos way back in this thread showing the inside of the base.

There are quite a few photos way back in this thread showing the inside of the base.