I bought a speedway one that came with adapters that fit it to flathead crankshaft--- not the best accuracy as checked on rotary table--- the "printing"was a little off center, but not much more than the width of the printed line
Kurt, the only one I could find at Speedway that fit a flathead was the Offenhauser one that had the crank hub part of the wheel. Flathead crank is about 1.310" What did you find?
You run the chance of it loosening when you turn it backwards.
R