Yes I'm not sure what's going on with the front hub - but the bike is certainly the 4 h.p. (550cc) Triumph Model H. It used the 3-speed Sturmey Archer CS (counter shaft) gearbox, with belt final drive. I recall there were 25,000+ of these built for the war effort. At the Armistice in Nov 1918 there were 17,998 total on the books - 9,813 "overseas" and 8,185 "at home" (in the UK).
The petrol tank was a silver-grey (my guess more grey on the military models - not much of an advantage to glitter) with green panels. The rest was black, with small parts nickel plated.
What's the book about Matt?
Cheers
Leon