Thinking it went to some sort of pipe shut off valve thing.
I agree, Hosman, and I'm thinking it was a water "shut-off" valve thingy.
And I'm also thinking that since it was made for a square shaft/stem, that it was not permanently affixed to the valve but was only used when needed. A "tool" one carried with them.
A "tool" like for "turning"
a fire hydrant "on" & "off"
by only firemen or public employees, and that would be why it has "OPEN" on it cause fire hydrants are always "CLOSEed" or turned "OFF".