Hans,
There are pre war service sheets and manuals like Pitman's Book of the BSA and D.W. Munro BSA motorcycles. Furthermore there is a manual by Motorcycling or reprint by BMS called BSA Motorcycles 1935-1940. These can all be found on Ebay. Please note that the Pitman's Book of the BSA comes in many editions. The most common one stretches from 1936into the late 1940's early 1950's. Earlier editions are more limited in scope of models (1st up to and including 9th edition).
Kind regards,
Leon