Apologies if these questions have been asked and answered before.
Can anyone please tell me the answers to these queries about the personnel record cards in AIR 78?

-- What does the abbreviation N S A mean? (Possibly National Service Airman?)
-- What is the meaning of the word "Militia" which appears on some cards? (Sometimes typed, sometimes rubber-stamped.)
-- What does the rubber stamped F in a circle mean?

Thanks in advance.