Half of a gate valve gate.
Notice the "wings" on the side of your piece. Two of these pieces would fit together and the "wings" are what traveled in a slot
as the gate was raised or lowered to open or close the valve. Notice the half circle indention at the top. This is where one end of
the valve stem would go between the pieces and the other end would attach to the valve wheel. Also notice the ball shaped
feature in the center. The mating half would have an indention for these two pieces to line up right.
Here is a modern version of what you found.
