What I don't understand is why did they need to advertise
You had to get a lot of bums through the door in order to select the best steely-eyed heroes for the front seat.
In the late 70s when I was on recruiting the officer and aircrew ads also had a flow-on effect for the very high requirement for ground trades. Many was the starry-eyed lad that came through my doors wanting to be a Harrier pilot and was signed up for RAF Policeman (and even then we probably had to fudge the aptitude scores to meet the target!)