Here's the thing: 
This is not a Douglas motorcycle.
The engine could be one.
What brand is it?

What about ABC?
https://abcroadmotors.co.uk/history.html?start=1

We have also seen here previously that Duggie flat twin engines were used during WW1 to power generating sets
for wireless transmissions. And the opposition used something similar - Douglas copies, IIRC ?

And the flat twin ohv 350cc (or 500cc ?) engines were used circa WW2 and later for generator sets also.
Don't recall if these were Douglas built, or supplied for this purpose.

I'd comment that I started my Douglas 'career' with a 1920s 2 3/4 engine, that had been adapted for use in a small boat.
But this was post-motorcycle use, so doesn't count. Don't know how many of these were done, bit homemade.

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I have this WW1 Douglas generator, well half of it. The dynamo, fuel tank and radiator are missing also the base has been cut short. I'm also missing the carburetor.
Am looking for help.
Does anyone have a complete example? Photographs, drawings? What make and model of carburetor do I need?
Ideally I will be able to rebuild this for inclusion in a WW1 Leyland mobile workshop I'm currently restoring.
Many thanks
Doc

Postwar, Douglas sold a line of vertical, single cylinder, 4-stroke industrial engines that were used in UK based mowers, tillers, cement mixers, etc. And between the wars Douglas manufactured light aero engines for several small (and short lived) airplane manufacturers. As for supplying other motorcycle manufactures, none come to mind. Even the Williamson engine I think was made bespoke for Williamson. Maybe they had thoughts of using the same engine in one of thier own light cars, but that never took off. 

Offhand, the Williamson comes to mind, for a preWW1 outfit (sidecar machine ) - an 8hp job  (964 cc ?).
It helped that the Williams bros were cousins of William Douglas
And with all those Williams in the plot, it had to be 'Williamson' !?

Didn't BMW pay some sort of royalties to Douglas. ?
Without ever actually using Douglas engines. ??

Fairey and Douglas were somewhat intertwined in the very early years,
but it would be fair to say it was complicated ?

If there are more examples, they are not leaping out to the AI bots, or me ....

Some makers, like JAP and then Villiers etc etc supplied engines to numerous makers.
This was entirely their business model. But not flat twins ...
Hopethishelpsabit.

Hello. Is there a list of manufacturers who have installed Douglas engines?

Specifically 2 3/4HP, 4HP and 6HP?

Thank you

Thats the cush-drive hub, where you can see the thread missing in the outer few mm. Not like the one you posted in the other thread.

Hello Doug,
took it into parts today and searched for a spot to get the round about 8mm I need. You will get 1 mm here and 1 there, but all not stisying.
Best way to put a spacer behind the sprocket, but unfortunately the first some mm of the cush-drive hub thread were cut off. Before I weld on and machine it... I may ask if there is a chance to get a  cush-drive hub with the full thread till the end.