Michael, I now have two Myford studs, they are identical, one came with the lathe the other I bought recently but it proved to be too short for comfort for my new tool post. So I bought a 7/16 UNF bolt to make a new stud and found that the nut did not fit. At my local excellent fixings supplier, a quick measurement showed that the Myford stud was 18 tpi, and it clearly has a deeper thread form than unf. Hence most likely to be BSF. The nut supplied measures 17.95 mm a/f, 11/16" a/f is 17.5 mm (which is what they say the wrench is), while the spanner supplied with the lathe is "3/8 whitworth " and is just over 18mm a/f, actually the recommended size for 7/16 BSF.
UNF nuts seem to be 5/8" a/f, and the thinned down head of a 7/16 UNF hex bolt fits snugly across a 16mm wide pocket milled in the base of my toolpost block.