Registration Cards
Danny
The point I was trying to make was that the Identity Card formed part of the September 1939 registration process, with a card being issued to every man, woman and child on the census record.
Registering for National Service was an all together different process requiring men to attend their local employment / labour exchange on a specified Saturday based on their DOB. They would receive their National Service registration card and then be instructed to submit themselves for a medical before being placed on the register for those available for service with the armed forces (as you describe in your post)
I hope that clarifies
Regards
Pete