On close inspection, the cycle parts look to be predominantly pre WW1, but the engine is almost certainly later (one piece timing cover). How's this for a thought? - it is well documented that Douglas took back a large number of ex-WD machines at the end of the war and refurbished them as civilian models. Presumably, these machines would still have had the date of manufacture as 1913-14, but could have had post 1918 parts fitted (i.e. the engine). Doug thinks the rear carrier could be 1916 type, but from the catalogue pictures I have, this item looks more like the style fitted in the early 20's - but this, of course, may have been fitted at a later time. Most of us don't put enough detail on the backs of our photos - at the time, the minor detail seems obvious, but years later, doubt arises. So "Dad's Old Douglas Bike (1914)" could refer to the year of the bike, the year of the photo or, possibly, the details could even have been added at a later date! (in which case it was probably just a guess).