Yesterday's Find: Superior Pittsburgh, PA. wrench

[Wrenchmensch]

I found this wrench in a tool box at our local flea market. It is five inches long, its open end is 1 1/4 inches wide, and its closed end is a 3/8 inch square.  One side is embossed "SUPERIOR" with a "0" embossed to the left.  The other side is embossed "PITTSBURGH, PA."

I don't know what this wrench was used for, or went with.  Can anyone identify this wrench?

[oldgoaly]

1st thought was a cylinder cap wrench, but the sizes are not right for the cylinders I'm thinking of like oxygen, acetylene, refrigerants. maybe ammonia refrigerant?