Incidentally, very early in the history of "u-ground" devices, there was a grounding t-slot receptacle manufactured! I have a picture of one in a 1952 catalog. I call it a NEMA 5.5-15R. I've never seen one of these. I bet very few of them were ever made. I'd just about give my eyeteeth for one. One of these days, I'll get my scanner hooked up and post the picture.
