It looks a lot like a handmade version the water key I remember my father using for the sprinkler system at our house. I’m assuming they were used up until electronics and times came into play with the sprinkler system. Do you have a sprinkler system in your back yard that is on a timer or is controlled electronically? I’ll bet this was just discarded and forgotten about when your or whoever lived at your house at the time switched over to a more high tech sprinkler system.
