Designed and engineered by the U.S. Postal Service, these Rack Ladder Mailboxes correspond to the USPS series modules. These secure P.O. boxes can be installed in a wall between Rack Ladders or in a Rotary Enclosure. Five mailbox units per column (requires two rack ladders). Each additional column in the same configuration requires an additional rack ladder. Great for universities, postal centers, hospitals, mailrooms, and apartment complexes. This convenient lockable P.O. box unit comes with 4 tenant doors.