Strange receptacle, oxidation on ground prong for washing machine?

So I've been going around replacing receptacles in my 1972 home. Today I focused on the laundry room, which is on the main floor of my house.

When I pulled the plug on the washing machine, this is what I found:


This is a 20-amp dedicated circuit, properly wired with 12 AWG. The washing machine is rated for 12 amps; I sometimes plug a radio into the second outlet. This one receptacle is the only thing powered by the circuit. My first thought was moisture causing the oxidation, but the box is bone dry inside.

The other strange thing is the receptacle itself. The openings for the ground prongs go right through to the back of the receptacle. I shined a flashlight through in this pic for clarity:


Here's another photo showing the rear side of the receptacle:


I've replaced several receptacles throughout the house and don't recall seeing anything like this one. Any help or advice greatly appreciated. Thanks!

 

If there is no moisture then all I think of would be to check the other end of the cord and make sure the neutral is not in contact with the ground. Maybe there is current flowing on the ground and causing the discoloration. Just a theory....

 

The only thing I can think of is galvanic action caused by the brass ground prong toughing the steel yoke.

 

So the openings for the ground prongs on the recept. that go all the way through don't have anything to do with it? As I said, I've never seen a recept. like that before ...

At any rate, I just finished swapping out the receptacle with a brand new one. I cleaned off the oxidation from the washing machine plug with my Dremel and the little rotating wire brush. I'll keep an eye on how it does.