Readydale
I had an idea that the link to the area codes table was somewhere in that earlier thread, but I couldn't face the prospect of trawling through 30+ pages to check!
Most numbers consist of the 3-letter area code followed by an enumeration district letter, as you say. The exception is where a card was lost or stolen and a replacement was issued, where the last letter was dropped and Y was added to the start (there were no area codes starting with Y). Quite a neat idea, really. You can usually see the 3-letter area code on the office stamp on the blue cards.