Part of the hammer or whatever you were hitting on.
Joevalk- You got it.

I just went and checked and it is the base of my Snap On hammer that I guess I was using to beat on the distorted nut cage. Obviously it was not intended for such use...

I'm glad to know that it isn't a plug for a wire junction box up under the fender or something along those lines.
Butt plug...hahahaha. Nope.