The usual place this is seen is on the index cards in AIR78 which are free download from the TNA.
It means that his entry to the RAF was via the Military Training Act 1939
https://en.wikipedia.org/wiki/Militia_(United_Kingdom)
"The term militiaman was briefly revived in 1939. In the aftermath of the Munich Crisis Leslie Hore-Belisha, Secretary of State for War, wished to introduce a limited form of conscription, an unheard of concept in peacetime. It was thought that calling the conscripts 'militiamen' would make this more acceptable, as it would render them distinct from the rest of the army. Only single men aged 20–22 were to be conscripted (given a free suit of civilian clothes as well as a uniform), and after six months full-time training would be discharged into the reserve. The first intake was called up, but the Second World War was declared soon afterwards, and the militiamen lost their identity in the rapidly expanding army. "
Nothing sinister just one of the systems to allocate manpower to the services that the treasury maintained and needed accounting.
Found my post on NSA and F in a circle marks
https://www.rootschat.com/forum/index.php?topic=113665.18Ross