The forage cap was often a sore point with gp capts, especially stn cdrs. They didn't like wearing the same headgear as the junior officers.
At Akrotiri early 70s, OC Bomber, M*** B, always wore an SD hat until there was a big security exercise. To avoid being kidnapped be borrowed a forage cap.
At least with an air officer's cap they are ready to join the airship club.