There were other push-rod-return spring designs at Douglas after the 1920 TT and before the DT:
Yes I think some later TT bikes (1927?) had push-rod return springs before the DT. I will look for a picture.
Re the MAG design: The Motosacoche headquarters and factory was in Switzerland (Motosacoche, Acacias, Geneva, from memory), so presumably all development was done there.
Could be, information is scarce. The London M.A.G. factory at Willesden Junction, was not that far from Matchless at Plumstead as the crow flies and De Lissa had earlier links (1909?) to the Collier Bros?. Will try and dig a bit deeper....
I was wondering what other components used in the 3 1/2HP Sports engine we could possibly find the origins of. Seems like the gudgeon pin securing copper domes and the basic crank detachable counter weight setup was used on the earlier 3 1/2 HP side valve spring frame model as described in The Motor Cycle 1st May 1919.
This gudgeon pin securing method may have only been used for a short time on the S1 and replaced by a different method (using a split pin). Also the detachable counterweight securing method appears to have been altered for the S1 , but the basic principle is the same. The method of retaining the big end rollers was altered for the S1 tho'.
The Motor Cycle article from 1st May 1919 also appears to have provided an answer to which model Douglas, the engine picture on page 203 of Clew's “The Best Twin” ed 2 came from - maybe not a Williamson?!!
So the 3 1/2 HP sports engine appears to be an amalgam of designs from a few different sources with contributions from Bailey,
Pullin, Moore, (Edit) W.W. Douglas, design draftsmen like Curtis and many others.
1st May 1919 wasn't long after WW1 finished and maybe indicates that Douglas were possibly working on the design of the 3 1/2 HP side valve spring frame machine through the war years??
cheers
Hutch