Ahhh, so that's the explanation of the oddball size by today's standards. I had a look on a chart on this page:
http://www.dummett.net/ime/website/i...ner_sizes.html and it shows a difference of under 2.5 thou of an inch between the 28mm and the 5/8 Whitworth/11/16 BSF size. That difference can more than be made up by the layers of paint on the nut.
Just checked the NAPA page, and no Whitworth is listed, although of course they can be had form ebay and elsewhere. I think I'll be ok with the 28mm.