Anyone keen to follow up non-recreational pharmaceutical use use might, I think, make progress through a contemporary but secondary source lurking in AIR 57, in which are to be found the minutes of the Flying Personnel Research Committee (FPRC). This organization started life in January 1939, and was at that time much concerned with the use of stimulants. They were very much aware of Wehrmacht practice by the summer of 1940, and I imagine that the discovery of Pervitin in a crashed a/c would have been communicated by the relevant bit of AI to FPRC and is likely to have attracted a passing reference in their meeting minutes. A useful tour of the scene is Pugh, James (2011) ‘The Royal Air Force, Bomber Command and the use of Benzedrine Sulphate’. Journal of Contemporary History.
https://doi.org/10.1177/0022009416652717. Footnotes are useful with TNA references. Good hunting.