My theory as to why they used British Standard thread is the Victa two stroke came out in the 50s when were still attached to Britain's apron strings, and the motor just kept evolving from the same crankshaft and they just stuck to the established nut and thread even though we were moving away from British imperial thread systems.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!