Rudge-Whitworth splined hubs and knock off spinners are made in many different sizes. All MGA (and MGB) use 42-mm series hubs and nuts, but the thread changes from fine to coarse. The wheel type designation refers to the maximum size of outer bearing, in millimeters. Actual hub diameter is measured across the outside of the splines on the car, not the wheel.

Wheel Hub Dimensions (mm)

Wheel Type	Actual Hub Diameter	Number of Splines	Spline Length Short Hub	Spline Length Long Hub
35	52	62	36	56
42	62.5	75	37	62
52	73	88	37	62
62	82.5	100	57	78
72	92	112	55	84
80	102	124	58	87
90	111.5	136	56	94
100	123	150	59	97
120	137	168	63	101

In Europe most splined hubs are of the Rudge-Whitworth type having V-shape splines with dimensions as above. On Rolls Royce and in America, you may find RAF type hubs which have square splines.

I now have an example of a splined hub that has metric threads for the knock-off nut. They are not 12 or 13 threads per inch. The threads are 2.00-mm pitch, and 52-mm diameter (not sure about the spline size yet). Apparentry not Rudge-Whitworth standard. These small hubs are from a 1964 ASA 1100 GT (a small Italian car, go ahead and Google it).
Photos compliments of Gary Bohrman in Great Falls, Virginia, USA.

More spline measurements from Gary Bohrman in Great Falls, Virginia, USA.