Seen today in Foyles bookshop, London, is a new book called "Warpaint: Colours and Markings of British Army Vehicles 1903-2003" by Dick Taylor.
This is the first of 4 volumes. The author's preface suggests that, in time, all four may be combined into a single volume. No doubt that will depend on expected sales and new information coming to light (the author is soliciting for further information / corrections).
Unfortunately, the annexes on WW1 numbering systems provide little in the way of additional information for this string. However, for the first time I've seen in print, it does quote (albeit unattributed) General Routine Order No. 944 of 26th June, 1915, that:
"All motor cars and motor lorries must have their official registration number painted on the bonnet and also on the back of the vehicle. This number is to painted on both sides of the bonnet in white paint, in a conspicuous position at the back where it is not likely to be obscured by the tyres, tarpaulins or other obstructions. The size of the figures in all cases is to be as under:-
For motor cars - 4in. high, 1/2in. wide
For motor lorries - 6in. high, 5/8in. wide
A broad arrow is to be placed on top or on one side of the numbers, according to the shape of the bonnet, as may be found most convenient. No numbers or letters are to painted on the bonnets or on the backs of the vehicles except the official registration number."
The inter-war system gets a mention, but doesn't really add to what can be found in Hodges.