Hi Manfred,
Doug has answered your question, I am curious how the engine ran without the locking nut!
The Douglas Club does have copies of both the original manual and the parts list for the S6 models, they cover T6 ,S6, S5, C31, D31 and E31. Sadly a few of the pictures have suffered from early photographic techniques but they do show all the componants.
Except of course your missing nut! it is listed in the part numbers as 11163. It would have cost you the princely sum of 3 old pence! in 1931.
Should you not already have copies, the club sells them for £7-00 each plus postage. As treasurer of the Club I am quite happy to send you one or both and collect your money!
If you would like to order please write direct to my email with your address.
Dave