The HUP was originally fitted with 9.25-16 tyres, which
comparing some current day survivors, was apparently smaller than a 9.00-16.
Could it also be that the 6 stud rim for the 9.25-16 is built to the US 16" rim dimensions, while the 8 stud rim for 9.00-16 is built to Cdn/UK WD 16" dimensions?