What the hell is that?

When we moved into our house five years ago, one of the first things I did was replace some of the more out-dated electrical outlets and switches.  I came across this one in my living room.  I have never seen an outlet this old.  It looks like it was made from molded sand, which falls apart when you touch it.

I recently had the opportunity to show it to a friend on mine who is a home inspector.  His only response was "What the hell is that?".