Hello Bill
The finning on the barrels above the valves show this to be a 4hp engine - so your thoughts of it being 1915/16 are probably spot on. The gearbox fits on the sloping part at the rear of the crankcase, so has a matching angled extension. Hopefully the clutch will still be attached to the box - either a cone clutch within the pulley, or a plate clutch on the primary side, I'm not sure when the change in clutch type was made. The wheel hubs will have Douglas stamped on them. The magneto is likely to be either Bosch or CAV, though I have seen machines fitted with Fellows and Ruthard - who knows what the history is here. The carburettor could be Douglas, with separate air and fuel slides with an oval aluminium domed cover, cast together with the float bowl. It could also be a AMAC. The rear brake will likely mount forward of the belt rim, wedging into the rim. The 3 1/2 machines have the brake operating on the back of the rim. I can't wait to hear what else turns up!
Cheers, Graeme