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An Oakland "8" Tappet wrench. Model S8267. Only shows up in the 31 cat. A special short handled wrench with 3/8" and 1/2" openings, designed for valve tappet adjusting work on 1939 Oakland "8" models. Equipped with a chain and ring for looping around user's arm or attaching to the radiator rod, so if the wrench slips our of the user's hand and drops into the crankcase, it can be easily recovered.
Other than ignition wrenches, probably the shortest BP wrench. I had to add the chain and loop.