I would say it is a socket tool for a brace, or a brace like tool. With the cut out, it may have been used in wagon manufacturing back when. The wood farm wagon box was actually made rather lightly. To haul a full load of grain, the box had to be held together front, middle, and rear with reinforcing rods. 3/8 inch rod, with a loop on one end, and threaded on the other. Instead of a plain square nut, it used a winged nut, with a 3 inch wing on one side only. I can see where this socket would work to tighten this wing nut. Am thinking manufacturing, rather than a farm tool. I try to collect brace sockets.
