noun
Post Office box number; P.O. box number
Used in addresses to specify a P.O. box number, often in computing contexts for address fields.
私書箱番号を入力してください。
Please enter your P.O. box number.
私書箱 refers to the P.O. box itself, while 私書箱番号 is the number assigned to it.
Compound of 私書箱 (P.O. box) + 番号 (number).