weirdly wired outlet burned

My son smelled smoke and located it coming from the outlet behind and used by the refrigerator. When he pulled it out, he saw the one side of the receptacle had melted. He pulled everything out of the box and called me, a DIY wirer, for advice. Even after he sent me photos and a diagram, I had no idea why it was wired as it was, or why it had shorted. 2 wires entering the box are plastic insulated and the other 6 are fabric wrapped. I'm attaching photos and a diagram he made. Two fabric-wrapped black wires were attached to the outlet, he believes to the brass screws. The black plastic coated wire was in a silver push-in, and the the white wrapped was attached to a silver, where the insulation had been stripped. We're wondering why it is wired as it is, why it shorted, and whether all the wrapped wiring with no ground will have to be replaced. When it was inspected before being bought the inspector said it was 50-75 years old.wires pulled out of box

[!wires pulled out of box []2