I doubt very much those are UK or Trade plates.
Trade plates had a red back ground, the ones I used also had a triangular license holder. Further more they were never painted on. Nor were they need on the front of a bike,the whole idea was that they were a temporary useage, not permanent.
In the era of those photos most Uk reg were still on two letter and 3 or four numbers format.
The hyphen was often used between letter and number, but never to seperate letters, and indeed is illegal today and back then; number plate lay out has always been strictly set out in law, and despite todays fad for personalised reg plates, if set out in an unorthodox way are an MOT fail.
The was no "code" the digits relate always to the issuing authority, not the make of the bike.
I still think you are barking up the wrong tree, they re most likey french, both Douglas and ABC had strong french connections in the early vintage era.
Lastly I have for years had a strong distrust of information spread in some books, especially since one of mine own bikes appeared in a marque history accompanied by the most incredible mis-information about it. It has never been corrected.
[if this doesn't satisfy you you could always write to DVLA or the national motor museum]
also
http://www.cvpg.co.uk/REG.pdfOtehr net sources of that pic indicate its 1916/17 which I find odd considering it was the middle of WW1.
