The story
Literally, the number of a box into which mail for a particular recipient is placed.
Etymology from Wiktionary, CC BY-SA 4.0.
box number
Today it means: A numerical address for postal correspondence, managed by a company or organisation on behalf of the addressee, and used e.g. for convenience, if one does not have a suitable physical address for receiving mail, or to avoid giving one's real address (e.g. in a newspaper advertisement). Full definition →