OK, OK, here's the story. THIS WAS IN THE DETATCHED GARAGE AT MY HOUSE WHEN I MOVED IN!
(Sorry for shouting.)
Can you believe it? No, there was not an extender on the box. There was a duplex switch (am I using the correct term) on the right--a switch over a switch. To the left was a 3-way. This whole mess follows underground back to the house where the second 3-way switch is located.
There also had been a duplex receptacle further down the line, which had a strange characteristic: Plugging in a device in the top receptacle disabled the bottom one.
When I opened up the device, I found some spider eggs in there. I wonder if that had something to do with it.
Yes, it does appear that somebody spent quite some time putting it all together.
Here's how it was grounded:
to me that the bare wire is of a smaller gauge than the others.