FWIW Whitworth bolt heads are the same size as metrics. It would not surprise me if the diameters were metric as well. I have nothing anymore to check for thread pitch but I'm guessing that would be TPI. And metric threads are the 55deg thing too. At least on bikes and motorcycles. Sorta like one from column A, one from ..................

have another day
Dr.WtS


Dr.WtS
Mysteries of the Cosmos Unlocked
available by subscription