Thanks to the wonderful service at the VMCC library, I have the answer.
Thank you for your enquiry regarding number plates in 1938. By reference to the 1938 27th Edition of Motor Cycles and how to manage them I quote verbatim:- Front number plates must have the letters and figures 1 ¾ in. high, 5/16 in. broad and occupying a total width of 1 1/4in. except in the case of the figure “1” or the letter “I”. There must be a quarter of an inch between letters and figures, on a margin of at least half an inch between the outside figure and number and the end of the plate, and a quarter of an inch at the top and bottom. A space of one inch is required between the number and the index.
On the rear number plate all the figures and letters have to be 2 ½ in. high, 3/8 in. broad occupying a total wide of 1 3/4in., expect in the case of the figure “1” and the letter “I”. The space between adjoining figures and letters must be half an inch, and there must not be less than half an inch of margin between the nearest part of any letter or figure to the border of the plate.
If the letters are placed above the numbers there must be half an inch between the bottom of the letters and the top of the numbers.
I would just like to say again to the VMCC library, a huge thank you, you provide a superb service.
Jeff