Wasn't too sure what bike it is but sounded familiar; a search result for the 21-431 was Barleycorn and picture shows what appears to be a fine thread and without pulling mine off in the dark, I say it's 3/8 BSC
Maybe not much help for originality, but the collar 67-6078 is a square section, but don't remember the exact measurement, but think it's ~5/16" wide (it's critical), the ID sliding fit on the axle stub, OD just smaller than the chainwheel hole.
Sorry I can't help more than that without pulling mine apart
