The London Douglas Motor Cycle Club sell reprints of various spares lists, see their website for Shop items or any current issue of the Club magazine The New ConRod.
I had a look in my S6 spares list, and none of those numbers cross reference. Most of the new S6 part numbers are in the 111xxx to 112xxx range, so unlikely to find anything numbered much higher that far back. If Douglas could have reused it they typically did and kept the number. It might a gain suffix to indicate a revision, such as 11155-2 (S6 crank pin bush) that was otherwise backwards compatible. Getting into the mid-thirties models, spares lists seem non-existent and Douglas might not have gotten around to printing any. There were too many models and not enough money.
The 1934-35 Wessex and Powerflow were derivatives of the S6 engine, so probably had a crank pin bush to suit. The Aero crank was based on the Blue Chief/Endeavour engines; quite rare models. Those had oil fed through the crank, so the bush might not have been exactly the same; though it might have been something as simple as an aperture to let the oil out. Blue Chief numbers are in the 165xxx-168xxx range (roughly), so the locking tab washer dates from that time.
Generally, with the less prolific Douglas models, spares are only obtainable by cannibalizing or making your own (or paying someone).
-Doug
