THe handle is approx. 4 3/4" with what seems to be a hammer head at one end. The stem is approx. 4 1/2" with a 1/4" square hole.
It is marked A.F. Co
Possibly a piano tuners key/wrench.
Knowing the first part makes the second part easier...
A.F Co is likely American Felt Co, makers of felt for piano hammers....
Smart guys, I would be using it to turn off my Acetylene bottle!
If that wrench is for sale let me know.