In the GPO days, the Primary Cross Connection Point (P.C.P.) in the external cable network was known as a cabinet or CAB. It came in two sizes with the double door variety being most common. The two doors are held in place by two turnbuckle blocks secured by triangular headed bolts. The spanner to unbolt these was known as a CAB key.
A smaller capacity CAB with a single door being more common in rural areas. Both examples shown have the letters GPO on the lid.
RingBell : GPO Nostalgia Web site