In my older home, we have three interesting wall plates. Two are on the wall about a foot below the ceiling, and the other is embedded in the top of the plaster mantel. They are rectangular covers like a switch would have, but they have a single round hole in them. When removing the cover, there is a porcelain socket (with the hole) which unplugs revealing contacts. There is 120volts to the contacts. Two are switched, and the one by the mantle is unswitched. My electrician, who works in many old homes, has never seen anything like this. It appears some sort of light or device would have plugged into the porcelain hole. We want to make use of them for picture lights. Anyone seen these?