What type of outlet is in the picture?
My guess is that it is a 300 ohm terminal for a rooftop television antenna. I'm surprised it doesn't have any labelling molded in. If you pull it out, I think you'll find a flat 300 ohm (non-coaxial) antenna cable inside. I googled a bit, and found this thread where someone followed their attic antenna lead down to an identical wall plate.