Hi Roger,
Sorry, I'm not quite sure where I found the illustration
https://www.douglasmotorcycles.net/index.php?topic=7014.msg27401#msg27401 , but I wonder if it was from "Flawed Genius"? Maybe the ABC Road Motors website? I'm far from home so I can't check just now.
"described in the press in late 1912": The 500cc ABC motorcycle engine - with the three-throw-crank - was described in detail in the motorcycle magazines at the end of December, 1912.
For example, the Motor Cycle issue of 26 Dec 1912 that carried the story of SLB's record-breaking at Brooklands on page 1544 ("... At Bailey's request, Mr. Bradshaw designed and made the cylinders, valve, pistons, and connecting rods of an exactly similar type to those used in the new [ABC] aeroplane engine, and it was with these that the Douglas motor bicycle was fitted, experimentally, when the above records were broken...") also carried the story of the new 500cc ABC motor cycle engine on p1541.
I'll attach both below. My reading of articles like this, and many others in the period, suggest that Bailey was developing and racing bikes for Douglas in the second half of 1912, and got some assistance from Bradshaw to develop the ohv engine.
The report of Bailey's record-breaking Motorcycling, 24 Dec 1912, p252, contains a contradiction to this storyline, by stating in part that "... Bailey has employed his spare time in having built a very remarkable 500 cc horizontally-opposed twin-cylinder engine..." My views on this are here:
https://www.douglasmotorcycles.net/index.php?topic=7014.msg27403#msg27403 .
But clearly you have new info - please share!
Cheers
Leon