As I understand it, the second and third series were all printed in 1948. Been a while since I read up on them though.
They had had problems with non-military personnel obtaining the first series & decided (in 1947) to switch to the second series in virtual secrecy to limit conversion of the first series.
They also printed the third series so that if they needed to switch again at a later date, all the notes would be pre-existing so could be done more rapidly - though turns out they weren't needed until 1956.
However, I'm unsure of the introduction date. For notes of equal value, they were introduced in 1948 to replace the first series with immediate effect. However, there wasn't a first series £5, so could easily have been held back.
Numista uses the year of production in it's catalogue, hence 1948 is correct by those standards. Other sites/catalogues may use their formal release date instead which could be later - I don't recall & not sure where to look.
Hope that's been a little bit useful.
