What I don't know is where the letter is for direct entry officers before they are commissioned. I believe the case for those commissioned from the ranks the letter is moved from front to rear
PN,
I believe that is the case. I was attested into the UAS and was told that although we lived in the Officers' Mess, we were officially Airmen in rank terms. My service number started with a letter and then 7 digits. Fast forward a few years and on commissioning I retained the exact same digits, but the letter had moved to the end of the service number.
That would appear to fit your hypothesis, the only caveat being I don't know whether UAS numbers were out of the ordinary in that sense. I presume not as it would get hellishly complicated if UAS numbers differed markedly what with numbers for sponsored / UAS students, direct entry graduates, direct entry and females all having different numbers.