Having served on both squadrons the general rule was mod state and hours, 13 sq had most of the early,XH13? airframes and 39 had the later XH 16?17? airframes but that did change from time to time, the command spare XH176 was brought into service following the loss of a 13 sq airframe on approach to Luqa. 13's role was survey while 39 was tactical although both sqdns did a bit of both. Later after 13 went back to PR7's all the PR9's were put through a long SRIM programme and got mixed up and reissued to 39 over a long period of time. So no hard and fast rule, you got what you got!!