If I recall correctly, Whitworth wrenches are marked for the bolt/cap screw diameter, not the head diameter; and different thread pitches would use different head sizes for the same major diameter. So, if these wrenches were made by a British manufacturer, it may have been an attempt to inform both British mechanics, who presumably were in the habit of looking at the diameter of the bolt, and American mechanics, who would look at the diameter of the bolt head.
Every time I see a Whitworth wrench, I shake my head in admiration of the work habits of British mechanics, who would have to learn such a complex set of information in order to be able to get the correct wrench out of the toolbox.
