Below is the Series C set that I've recently obtained. These were exceptionally hard to get in UNC condition (the £50 in particular).








Wow, Mac. Very interesting read. I guess I'm off to buy your bookMac wrote: ↑11 Nov 2019 23:36E was the base letter for the £1 note in 1982, hence the Specimen with prefix EEE.
I have not seen a Specimen AAA for the £1 note. This might be a typo - need to see a scan or an example of a note.
L is an interesting prefix, as it was scattered, being used on £10 notes, and on £20 notes. I speculate that it might have also been the intended base letter for the £100 note. - I'd have to look in-depth to try and work out why it was used on the 1983 £5 specimen.
Have a look at the book, you might be able to work it out from there, as the sequences are listed on pages 121-125.
The prefixing system of £5 notes is also scattered, and was very complicated to record when we were doing it!
The choice of letters for the £10 specimens does not fit with other denominations.
CCC and HHH would have been expected, AAA and EEE were used! Your guess is as good as mine here.