Obsolete term for a post office box; replaced by 私書箱 (ししょばこ) in modern Japanese.
See also: 私書箱
Old letters were addressed with 'P.O. Box No. ...' using the term 私書函.
Compound of 私書 (private correspondence) and 函 (box). The character 函 is an older form for 'box', now largely replaced by 箱.