The item below was inherited from my father. But, first, to answer the unasked question: No, he never taught me anything about woodworking. Refused to. I learned to operate a broom and how to kill alley rats with bricks.
Anyhow, I'm sure this is some kind of tool used for squaring corners for mortises. It is spring loaded and the black part is a square cutter. I've never used it, and it looks unused, but I presume you drill a hole then place this in the corners and give a whack to cut square. It is 7/8"X7/8". Any further enlightenment would be appreciated.