If he joined 5 years ago, he joined as a WSOp first then a specialisation second. The old trades of Air Eng, AEOp and ALM were almagamated in order to give Manning the flexibilty to cover shortfalls in specialisation with the minimal amount of cross over training, hence the reason for the common core phase at 55(R) Sqn.
When he joined up it was with the understanding that he is joining a trade that had several specialisations, the one to which he was most suited/ service needs would be the one that he would be initially selected for, and can at any time be transferred to any other of the specialisations when required. This would mean that he would be joining a trade that spanned the country from Chivenor to Kinloss.