Synthetic
When a reconditioned Sunderland left the factory/MU some of the equipment, leak stopprs etc, were contained in aluminium suitcases.
These were large brown painted with a strong leather handle and stainless steel locks. A squared off shape with vertical sides. As they were non inventory items they were claimed by the ferry crew.
The story goes that Shorts enlarged the shape attached wings...........
The Skyvan !