The Mail Pouch
CSOS Gilnahirk.
From an interview with Mr N Harvey, former member of admin staff.
At the end of each working day a heavy leather lockable pouch was taken from the wireless station to the central Post Office in Belfast where it was handed over and signed for by a member of the Post Office staff who was a Special Branch officer. He then took this pouch to the Sea Link terminal at Belfast Harbor and boarded the ferry sailing overnight to Heysham. On boarding the ferry he was met by the ships purser who signed for the pouch and together they placed it in the ships lock up, a secure cage for the transportation of item of value and importance. On reaching Heysham the pouch made its way to "Oakley".
Each morning Mr N Harvey would repeat the process by collecting a pouch that had been sent from "Oakley" to Gilnahirk on the overnight ferry from Heysham.
As for the items / paperwork / documents in the pouch, that remains a mystery.