I found this as part of the description of a key on eBay: "The clip holds the cover closed, and can also be slipped over the skirt of the knob to hold it down and so send out a continuous signal. This allowed the radio operator to parachute out of a damaged plane while still sending a homing signal for rescue craft." I doubt that is why many non-military keys have a grounding bar/switch, but it might explain why some do... -D