Hi Ueli,
The complete engine is definitely from the OB range the engine number places it fairly late in the production the highest number we have in the Douglas register is in the low 800X range making it 1924, if you find a frame the correct number should be HF with just three digits anywhere between 100 and 950.
The pile of bits is possibly the remains of an S1 model, if you care to give the engine number from the crankcase we could tell you more precisely, if you prefer you can PM me directly via the email ikon.
Good luck with the hunting!
Dave