I'm often asked how I find such rare motorcycles; the answer is being ready to buy when the opportunity arises, and by keeping my eyes open. 
I found this 1923 ohv 600cc Douglas racer on ebay of all places. I was the only serious bidder; perhaps no one else recognized it for what it was; I knew that an intact overhead valve racing bike from the early 20's was extremely scarce proposition, especially in basically unmolested condition. I was reminded of my purchase of a 1925 Zenith supercharged JAP twin - similar condition...a little rusty on the surface, yes, but Australia has a dry climate like Southern California, and metal might have a haze of red, but it doesn't grow real rot, and can easily be cleaned up.
The seller knew the bike had been raced on the dirt tracks near Sydney in the 20's and was able to provide a photograph of the machine in the day - ridden by a fellow named Ted Reese. I've subsequently found a photo of an identical machine, ridden by an L.C. Peterson; the bikes are so close in spec and geography that I have to think they are the same machine - that droop of the silencer is distinctive.
Both photos were taken near the Newcastle track - Peterson is shown after winning a race on his Douglas. The OC engine of 600cc is from 1924, and would have been a capacity increase, and/or a spare engine!
Douglas was almost alone in 1923 in offering an overhead valve racing machine; almost universally among other manufacturers the norm was a sidevalve engine, and the overhead valve system was considered fragile and unproven. Douglas led the way with successful efforts at Brooklands and the Isle of Man,
winning the senior and sidecar TT races in 1923 with machines very similar to this bike (Norton's Model 18 was also introduced in '23... and they fetch astronomical prices). TheOB/OC used a total-loss oiling system, with an oil pump driven by the camshaft (inside the airbox). It uses two Amac TT carbs, which are linked by a rod system for synchronized slide movement. The airbox was a Freddie Dixon innovation - he reckoned that motorcycles would breathe better using a 'still air box', rather than sucking from a swirl of moving air. He was right, of course, and big ugly airboxes can still be found on Buells for the same reason; they make better power with them fitted. The airbox also makes a convenient air filter housing for dirt-track racing, which must have increased the longevity of the piston rings... Douglas made their own 3 speed gearbox, and the clutch is housed within the flywheel. Two 'dummy rim' brakes, and an EIC twin-spark magneto complete the picture.
