box number

noun

the number of an individual pigeonhole at a newspaper to which replies to an advertisement may be addressed
the number of an individual pigeonhole at a post office from which mail may be collected

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012