As I understand it the British marked their spanner/socket sizes based on the shank/thread diameter of the bolt, not the distance across the flats (A/F)
I believe that the distance across the flats was arrived at by applying a ratio to the bolt/shank/thread diameter which usually ended up being an odd sort of in between measurement for the distance across the flats.
The Americans went the simpler route and rounded head size to the nearest 1/16"( mostly) and marked their tooling accordingly.
David
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Hell no! I'm not that old!
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