Given that the requirement for grounding receptacles first became universal with the 1962 NEC (and universal enforcement probably took awhile), I have no problem believing the T-slot pictured here is from 1962. I don't know what year they stopped making T-slots. They seem to have been phased out in the '50s as industry standards changed to differentiate between parallel slots for 125V and tandem for 250V, but I don't know when they became illegal to use. I do know this: somewhere, I have a 1996 Leviton catalog listing NOS T-slots.
Coincidentally, the oldest part of my church was completed in 1962. I think there are still one or two T-slots in that building.
[This message has been edited by yaktx (edited 01-17-2007).]
