Ian and Leon,
From the 1915 Preliminary List "Douglas 1915 Improvements" (curtesy of Doug K.) the model U was a touring model with footboards and 2 or 3 speed, no clutch. It had valanced mudguards front and rear. The model V had dropped handlebars, 2 or 3 speed, no clutch, footrests and non-valanced mudguards (the 2 speed model would be more or less your "typical" 1915 dispatch rider bike). The model W was the top of the range, 3 speed, clutch, footboards and valanced guards and the model X was the ladies model which in 1915 was the same spec. as the model W (the 1914 model X being 2 speed with clutch).
Although 3 speeds were offered in the preliminary list for 1915, I don't think many 3 speeds were made between 1915 and 1919 - maybe less than 600 or so?
The carrier and a few other items on the bike in the picture don't look 100% original for 1915.
cheers
Hutch