Great. It shows my understanding of the numbering "system?!" is improving - slowly but only in part (1948... does not equal 49th Entry but 47th - at least at Hereford?)
It has been investigated and there is no direct relationship between Service numbers and Entry numbers.
Other than, over the years they both incremented!
I seem to recall that the letters were added to bring all airmen's numbers up to 8 characters as part of the introduction of computer records.
The alpha character that was added to service numbers in 1965, was indeed because of the introduction of computers. The alpha character was a 'check character'.
If a service number was incorrectly entered into a computer, it would produce a different check character. When compared to the original check character it would indicate an error.
The system was not 100% fool proof, but pretty close.
The extra digit may well have been added to former apps numbers to create a constant field length. It makes sense.
I was demobbed Feb 65 so never had the alpha character added, but I did then enter the world of computers and quickly learn't what a check digit was.
By the way, most check digits are numeric, as in you bank account number. In civil life I never came across a check system that used alpha characters.
As for guys called Bond getting the last 3 of 007, there appears to have been several cases of clerks making the unsuspecting guy wait until the stamp getting to 006 and saying, 'Right its your turn now!'