Richard,
Sorry if I caused confusion by calling it a "sales" booklet I meant the 1910 Douglas Manual, so it is probably the same one you already have?. I only know of the one 1910 document but the one I have is a copy of an original that was loaned to me, not the version from the club so I can only guess that they are the same. In the early days Douglas combined their sales information on the models for sale, with owner testimonials, results of competitions and technical details etc. Only later on (1914) did they split off a separate parts / workshop manual from the sales booklet - probably because by then the booklet was over 100 pages in length!
Yes I think the reason they listed the last year model alongside the new model is because they had not sold all of the previous years stock, also I guess they could expound on the benefits of their improvements over the previous year.
cheers
Ian