The tool is for removing valve port plugs in very early engines. Before about 1915, many engines had cylinder jugs that did not have removable cylinder heads. This was done because head gasket technology was weak and the headless design meant no blown gaskets. The engines were side valve or flathead design and to get the valves out of the jug, you unscrewed a plug in the top of the jug that was directly over the valve. The plug was sealed with a copper ring that looked like a big spark plug gasket. The bad news in this design was that the threads on the plug were on the combustion side of the seal and the plugs were a bugger to get out with the threads packed with carbon. The other bad news was that you had to do it fairly often. Doing a valve job every three to five thousand miles was common. The top of the plug was a castle type design with four slots in the top wall. Most engines had the spark plug in the bottom of the cup shaped plug. You took out the spark plug and placed this tool in the four slots. Then you screwed a sort of a bolt down through the tool and into the spark plug hole to hold the tool in place. Then, sometimes with a little torch heat, the plug could be unscrewed. You can sometimes do it without the screw, but often the castle on the plug gets chewed up making it harder next time. The photo below is a Bulldog Mack engine with a homemade tool.
The tool in the photo was probably used with an air gun.
