In 1960 I was a student in the Royal Air Force at RAF Ternhill (on Piston Provosts). We had the last SBA system in the RAF. (The aircraft system was known as a TBA).
So, what on earth was that?
Well, it consisted of a radio range consitsing of "A" and "N" audio sectors but with the addition of an "Outer Marker " (audio "Oink, Oink, Oink") through to the Middle Marker to the Inner Marker ("Peep, Peep, Peep").
It was a difficult system to get your head around as a student but once you got the hang of which (audio) sector you were in when approaching the airfield then the rest was relatively easy.
I seem to remember that we came down to 400 feet or less.
I don't think you should doubt Ernest Gann.
He did lots of things that I could relate with and any youngster who is even thinking about taking up our wonderful career should read "Fate is the Hunter".
I became an instructor in 1963 and I am still instructing. I have loaned "Fate is the Hunter" to all of my really promising pupils.
They usually fail to return the book but I don't really mind that.
One good operator is worth one good book!