Looks like its a tube socket, that may have been made/modified to undo car aerial mast nuts. They have the mast going through the centre of the nut where it secures to the guard (fender). The nut has two opposing slots on it. I'm in the motor trade, and have used similar sockets. The "21" sounds like it was metric. I have also seen brass cap screw type bolts with a similar head. It was on some sort of old machinery or engine.
Jason