Designed and engineered by the U.S. Postal Service, this 12-door Rack Ladder Mailbox system corresponds to the U.S. Postal Service 2900 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.