Shackman, the letter follows for officers.
Shy Torque letters were introduced for all serving officers regardless of commissioning date.
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.
Also a lot can be gleaned from the initial nuymber too - 423 was direct entry whereas 608 was Cranwell - and so on. Changing branches, commissioning etc didn't alter that original number.