Private Mail Bag

A private mail bag, locked bag or caller service is a term for a special form of postal delivery in some countries, usually a government department or large private company, which receive large volumes of mail. Private mail bag addresses are often used in countries in Africa where there may be no street delivery service. 
Examples and variants.
Like PO Box addresses, private mail bag addresses omit the name of the building and street, and include only the number allocated to the user.
In some cases, a private mail bag may not have a number:
It differs from PO box delivery in that whereas the holder of a PO box has to go to the post office in order to collect mail, the mail sent to a private bag is dispatched to the holder by a mail contractor.
Private Bag.
In other countries, particularly South Africa, New Zealand and Fiji, the term private bag is used.
In Fiji the usage is similar to New Zealand
In South Africa, the private bag number may also contain letters.
Locked Bag and Caller Service.
In Australia, there are also locked bag addresses:
In Singapore, the term locked bag service is used:
These are similar to a private mail bag, but the holder must collect the mail at the post office, like a PO box.
In the United States, this service is called caller service. However, mail to a caller number is, with limited exceptions, addressed as though the caller number were a physical PO box, as seen in the following example: