In my 1970-era house there was a blank wall plate right where I would like a wall outlet.
I took off the plate and found a bare copper ground wire. Also two white wi res, two red wires and two black wires (all solid copper 12-gauge). The two black wires were fastened to each other with a "rubberized cap" of some so rt. The two white wires were fastened to each other the same way and the re d wires were fastened to each other the same way.
Called father-in-law who claims to know a lot. He told me it was set up for a 220-circuit and all I need to do was to unfasten all the wires, wire nut off one of each color and then take the remaining three wires (red, white and black) and fasten them as normal to my duplex outlet and reinstall into the wall.