Tr.9er (#11882),
An A.C.2. is an "Aircraftman, Second Class". or "erk", and as you surmise, the lowest form of life in the RAF and general dogsbody. Prewar and in the early part of the war, they were using up the last of the six-figure numbers. When I came in in Dec 1940, they'd got up to 1132877.
The six-figure men mocked our numbers (" that's not a number - it's the population of China! - "Get some in!" - and "when I joined, we didn't have numbers, we all knew each other!".
Of course, if you were commissioned, you got a new number.
Both numbers will stay with you till you die. (Is that true for Alzheimers sufferers, btw?)
Danny.