Upon further analysis, the ground lugs inside both sockets connecting to the 3 prong lamp were rusted out beyond practical use and will be replaced. IN addition, and to my disbelief, the ground connection going from the lamp ground pin to both (HI and LO) filaments within the lamp itself was cut on one of the bulbs...both filaments were OK (could be fed in series at low glow) but not connected to ground any more.

SO finally, it appears that the ground wiring itself was correct, and inserting a ground pin into the socket as I did in the first place just applied some force on the ground contact inside the socket providing "some" better contact...for a little while.