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David,
went out to the shop & dug up a spare new Vintage nut & gave a quick measure.
Taper-60 deg.
Overall length-.850
Dia-.750
Staight length ?-.735
Broached hex-12mm.-internal
Thread size-1/2"X20tpi.
Wheel hole depth-.750-.800

If these were made with a 1/4" tall hex-external, that would leave approx. 1/2" of purchase area in the wheel, would that be suffient?

Thanks Craig