Neil,
Well that and the Cotswold/Comet frame would be a "FY" prefix and the 1936 Aero frame would be a "AD" or "AF" depending if it were a 250 or 350cc. Not sure why, or what, the difference was between a 250 or 350cc Aero frame.
Between 1934 and 35 the engines lost the old style port gland nuts and changed to the flange mount intake and exhaust manifold. They also changed to the plate cylinder head. Here though is an oddity. Those features were introduced on the 1932 Bulldog as a lightweight 500cc based on a A/31 (350cc) bottom end. It used aluminium for the heads. Yet in 1934 the model Y1 and Y2 (350, 500cc) are shown with the older style detachable (non-plate) head and gland ports. Further, it is described as detachable head, and it is the 250cc model Y that is described and shown as having the plate heads. Also the flanged ports that all models would acquire the following year. Why the light 500 would revert to the old style head and the lowly 250cc get the latest plate head is a weird juxtaposition.
Anyway, between 1935 and 1936-37 there does not seem to have been any change in the engines. This is supported by the engine codes; the 250cc being 25/B for 1935-36 and the 350cc having 35/F for those two years.
-Doug
Identifying old Douglas motorcycles. Part 2: 1920s and 1930s (excluding 2-3/4 and 4hp).
https://www.douglasmotorcycles.net/aa-files/html/identify-part2/vintage4.8.htm