This tool is available commercially now but they weren't when I made this one. For the benefit of those folks who don't use milling machines the spindle has a drawbar which threads into the toolholders to keep them from being pulled out of the spindle taper when in use. To remove a tool from the mill one must first use a wrench to unscrew the drawbar slightly, then one gently taps the end of the drawbar with a hammer to pop the tool holder loose from the spindle taper.
This tool combines the two tools needed, hammer and wrench, into a single tool, the HENCH. On one end a (hidden) tenon has been cut to fit the square hole in the socket that matches the nut on the end of the drawbar. Said socket is held in place with a washer and a screw that threads into the tenon, all hidden up inside the socket. The other end of the hammer has a (badly oxidized) brass head silver soldered in place so that it will not mar the drawbar nut when it hits it.
So, unscrew drawbar with the socket, flip the tool over, and lightly tap the nut to loosen things.