Hi Paul,
The bike has obviously been restored at some stage, but it looks to be a really nice thing. Rebuild the front wheel with the correct rim and it would be a lovely bike, other than the engine dating issue. A lot of the detail is particularly nice, and mostly correct for late 1914 or 1915.
The interesting carburettor is Douglas, and similar to - but not exactly the same as - the one often found on the 1914 2 3/4. I wonder if it might be from a 3 1/2 h.p.? Perhaps someone can say for sure.
Numbers will be interesting. Douglas was a tiny manufacturer before 1910, but by 1913 the annual production was 5400. In September 1914 they had an order of 90 bikes from the War Office, a further 100 in October, another 200 in November... so regardless of what catalogues and magazines said regarding the specification, by late 1914 I'm sure Douglas were assembling whatever parts they had to hand. I think we agreed that something like 25-30,000 Douglas motorcycles went to war - mostly 2 3/4.
Of course once the bikes were in service, all manner of mix-and-match occurred to keep the maximum number of bikes in running order. No-one paid too much attention to what the catalogue said.
Cheers
Leon