It is of the B29-B31 350cc series; I would say the 1931 B31 model, due to the features of a offset petrol cap, and the adjustable method of mounting the handlebars. The centerstand replaced the rear stand in 1930. The serial number for the frame will be stamped on the lug for the rear down tube, right-hand side (sometimes it is stamped horizontal on the same lug.) It will have a prefix code of FA*** followed by three or four digits. Douglas often stamped their trademark superimposed D and K after the frame number, so do not mistake it for an extra digit. The engine serial number will be stamped on the top of the crankcase, to the rear, adjacent to the magneto. Look at the area underneath the carburetor. It will have a prefix code of EN*** followed by three or four digits. The gearbox serial number is stamped on the top of the gear case, adjacent to the joint for the cover. It will have a prefix code of RG/C*** or RG/E*** followed by three or four digits.