Hi Doug,
That's a good call about the inlet manifold and cylinders - fair to say the bike's not 1909.
Re the frame, a couple of snippets from The Motor Cycle of the day. The "low" frame for 1911 (as used on the Model E) was first seen in September 1911 when two Douglases, with two-speed gearboxes and the "low" pedal-less frame won the Sutton Bank Hill Climb and were promptly disqualified, since the bikes were not available to the public. There was heated correspondence, including from W. Wilson Douglas who pointed out that the bikes were standard 1911 models, for which orders were being taken.
On to the Show. Surprisingly the "standard" Douglas frame was the "old" (1910 pattern) frame with the pedal bracket. It was ready to fit the gearbox if required. To back this up, there were (wait for it) 21 Douglas motorcycles at the Show in November, and 18 of them had pedals. So there were only three Model Es on show - with the low frame, two speed gear, footboards, and clutch. Elsewhere it states that the clutch was an option on the Model E. I think (if you solve the puzzle from the numbers below) there must have been 15 single-speeders with pedals, three two speeders with pedals, and three two speeders with clutch and no pedals, on the Douglas stand in November 1910. Some were what we call "Ladies model", but it was made clear that the drop frame bike was suitable for ladies or gentlemen.
Back to the bike for sale - I'd be checking the frame number. I fancy it is mostly as it left the factory.
Cheers
Leon