It is a 4hp (600cc, 1915-23) model. The big brother of the prolific 2-3/4hp as typically used by the dispatch riders. It looks like one of Douglas' own sidecar bodies as well. There are a few folk on the forum that have these models that may be able to narrow down the year, particularly when you get a larger scan posted.
That pattern fork was available from 1916 on (which I think precludes this being a 500cc 1914-15 3-1/2hp), but the preceding veteran fork continued to be used for a number of years after, particularly on some of the the cheaper 2-3/4hp models.
-Doug