[anorak] Strictly speaking, the letters shouldn't appear after the numbers. The Air Ministry terminated the practice of placing the role letter after the Sqn Number in May 1939.
The practice began in 1924, but by 1939, it became clear that placing the role letter after the number had security implications (this is at about the time that the Air Ministry realised that naming every officer serving with a particular squadron in the Air Force List wasn't very good OPSEC), and squadrons that changed their roles would then (in theory) have to change their role letter.
This was fine in theory, but squadrons continued the practice. The Air Ministry repeated the 'stop doing this, will you?' instruction in 1941, with a bit more effect - but it materialised again.
The letter is now a 'heritage' thing. While 18(B) hasn't seen a bomber type since it disbanded on Valiants, 5(AC) (or V(A)C if you want to break another Air Ministry edict on the correct presentation of squadron numbering - no Roman numerals allowed, even if employed in the squadron crest...) will have the correct identifier for the first time in some years, having had little to do with Army co-operation when equipped with interceptors.
The main culprits for upsetting the Air Ministry have been II(AC), who took note of the edict not to use role identifiers, said, 'Mmm. Interesting idea,' and ignored it, before doing exactly the same thing when the edict against using Roman numerals was introduced.
Associations in the unit title, such as 257 (Burma) and 139 (Jamaica) were also kicked into touch by the Air Ministry. There were several reasons for named squadrons, such as Gift Squadrons (the place named providing the cash for aircraft) and a concentration of personnel from a particular nation within a unit, e.g. 44 (Rhodesia). The Air Ministry noted that this could all get terribly confusing, and that there was a danger of the named squadron's associated town/country trying to interfere in the allocation of squadron numbers.
They banned the use of names in 1952, and again in 1962, when the realised that some units had ignored them in '52. 139 still used the 'Jamaica' title up to disbandment, and the Air Ministry were proved right about interference. When the Victor B2 force was disbanded, the Jamaican government wrote to Harold Wilson (IIRC) and made representations that 'their' squadron should be reformed as swiftly as possible. The Jamaican government was politely informed that the RAF wasn't going to alter its policy on numberpalte allocation, but that there was a chance that 139 would be reformed in the 1970s when the Buccaneer force came up to full strength. Although the number was considered, more senior numberplates were reformed/re-equipped with the Bucc. [/anorak]