Ain't it pretty? All of the subsequent Douglas road racers - up to the 1932 TT entries - developed in a fairly linear way from the RA.
Here's my scribble chronology of S. L. Bailey as it stands at the moment:
1906 January 31, SLB nominates for the Sydney 1000 bicycle race
1909 October 28, SLB wins heat of 3-miles motor cycle race in Sydney on a Triumph
1910 February 26, SLB working for Canada Cycle & Motor Co., Ltd., Newcastle, racing (British) Excelsior
1910 March 24, SLB wins 5-miles Championship of NSW on his 5-hp Massey JAP
1910 August 24, SLB attempts records on Massey JAP and LMC motorcycles
1911 Feb 11, SLB racing in Sydney for the Newcastle MCC on Massey JAP
1911 September 2, SLB, "motorist", arrrives in London on the Otway, 3rd class
1912 May 11, SLB racing a 2 3/4 TT Humber in Birmingham
1912 June 1, SLB rides W. Douglas's competition machine at Brooklands in All Comers' One Hour Race: 56 miles, 755 yards
1912 Bailey rides a Douglas in the TT
1912 July 20, SLB racing a 2 3/4 side-valve Douglas at Brooklands (see higher in this thread)
1912 December "last week" (on 5th), SLB testing 2 3/4 OHV Douglas at Brooklands (with Granville Bradshaw parts)
1912 December 16, SLB due to leave for Australia
1912 December 17, SLB record setting at Brooklands on the OHV 350 (so departure delayed).
1912 December 19, leaves for Australia with Weatherilt.
1913 February 12, SLB in Forbes NSW, returning to Sydney to attempt records. Plans to return to England "for the opening of the racing season"
1913 May 13, SLB record breaking on 2 3/4 Douglas in Melbourne: one hour record
1913 June, SLB record breaking in Brisbane
1913 July 25, SLB leaves Sydney on S.S. Sonoma to UK via USA
1913 September 11, SLB back in England, quite badly injured in motorcycle accident (on the road, I think)
1913 October 4, Debut of "new" (narrow pushrod) OHV Douglas at Weston-super-Mare, Alexander riding
1913 October 11, "New" OHV Douglas entered for BMCRC meeting at Brooklands, ridden by Alfie Alexander.
1914 February 28, Track tests of "new" ohv Douglas (1914 TT model), F. G. Ball riding, SLB on crutches and timekeeping.
1914 March 28, SLB racing OHV Douglas at BMCRC meeting at Brooklands - 1914-TT-style Douglas engine, but in single down-tube frame
1914 September 22, SLB sets Class F (350 sidecar) records at Brooklands
1915 December 15, SLB "now holding an important position with Messrs. Douglas Bros." marries Cyril Pullin's sister
1917 May 3, SLB "Douglas", "well-known motorcyclist", attends Cyril Pullin's wedding
1917 September 20, SLB conducts a tour of the Douglas Works for "The Motor Cycle". Hints, but "not at liberty to divulge details"
1918 April 4, SLB and W.W. Douglas return from France, promoting 4hp Douglas for French army
1919 May 8, SLB injures hand in car accident in Bath at Easter
1919 August 29, Weston-super-Mare: F.G. Ball debuts 8-valve aluminium-cylinder Douglas racer - beaten by SLB on 1914-TT-style machine
1920 May 28, Tudor Thompson at Brooklands on 350 Douglas (presumably with "Sports" style motor)
1920 June, Douglas TT machines have "Sports" style motor with belt drive; brakes on rim at front, on belt rim at rear
1920 August 17, SLB beats flying-5-mile record at 66.18 mph: "Sports" type motor, chain drive, with precursor to RA/S1 disc brake at rear
1920 August 21, SLB sails for Australia with "a new 2 3/4 h.p. chain driven model"
1920 October 15, Back in Sydney for "thee-weeks' trip"
1920 October 28, Sports Model announced in The Motor Cycle with SLB-designed disc front brake etc.
1921 January 13, SLB "on his way back from the Antipodes"
1921 July 20, SLB applies for disc brake patent (granted as GB187687, 1922)
1922 March 23, C. G. Pullin exceeds 100 mph on 494cc Douglas
1922 May 19, Possibly SLB's last competitive race at Brooklands
1923 May 30, First public photos of SLB's masterpiece, the RA Douglas
1925 February 27, SLB arrives back in Sydney for "well-earned break"
In Australia 1925-1929:
Douglas Motors Ltd. factory representative for Australasia
Chairman of Directors, Penrith Speedway, Ltd.
Factory representative, Coventry Chains
Factory representative, AMAC carburettors
Vice President, Western Suburbs Motor Cycle Club
Vice President, Douglas Motor Cycle Club
Director, Missendon Road Motor Body Works, Ltd.
Director, Australasian Motor Investment & Finance Co. Ltd.
Director, Maroubra Speedway, Ltd.