The "extra" contact probably has something to do with controlling the performance of the charger. There could be a temperature measuring component (Like a thermistor, or even a diode.) inside the pack which tells the charger to turn down the charging current if things get too warm inside. One side of that sensor could be connected to one of the battery terminals and the other side to that "extra" contact.
I wouldn't worry about the 16.5 volts, that's about correct for the unloaded voltage of a charged up "14.4 volt" pack. If you measured that voltage when the drill was running, you'd find it much closer to its rated value.
Jeff