Prior to 1968 when the 8 series was introduced, if I remember correctly, the following categories applied
Beginning with:
......
On the officers' side:
60 Cranwell cadets (6 digits only) current CDS was in the last entry
26 Ex-Aux Air Force
52 University Graduates
43 Direct entry (ie, OCTU as opposed to Cranwell)
2X Females (number contained only 4 digits)
I am sure that someone will remember others - I look forward to being enlightened.