Hi Leon
I thought you might like be interested in to seeing my orphan, at least you have a machining number to try and possibly match your orphan with its other matching 2 parts of its crankcase. I have photographed my orphan which is definitely a 3 1/2 hp, unfortunately there are no machining/assembly number to match it with its buddies. I don't even know its engine number, it looks like a small 14 with larger 534, the larger numbers typical Douglas typeface. As you can see quite a bit of development has taken place over the years between yours and mine, the casting is much more substantial especially on the mating faces, the cam followers are now roller, the camshaft is more substantial, putting one against the other would probably throw up a whole heap more differences. Thought it might be of some interest to compare the two.
Keep up the good work, anything more in the wind on the R.A. saga, it has been really interesting up to now.
Best of Luck
Steve L