KOOKABAT
You got me interested in this subject today and a trawl round the internet suggests that the "21 lessons" was RAAF related.
There are several sites that refer to RAAF aircrew undertaking the 21 lessons whilst on deferred service and I found the following training manual c1940:
I am keen to follow up the UK side, so I have e-mailed the RAF Museum to see if they can shed some light on what was sent out to UK personnel whilst on deferred service.
I will keep you posted on that one but would love to hear from the veterans
Regards
Pete