hard to say without seeing it, but I would guess it was imperial and not metric, unless it has been modified,
depends how accurately you have measured it,
to convert 3/8 unc to metric you are looking at 9.5mm Diameter, UNC 16tpi would be 1.58mm pitch, and UNF 24tpi would be 1.06mm pitch, don't know if that helps you, or go to bunnings and pick up a couple of nuts UNC and metric and try them.
