Designed and engineered by the U.S. Postal Service, these Rack Ladder Mailboxes correspond to the USPS 2900 series modules. This 8-door P.O. box unit 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. These secure mailbox units are great for use in universities, postal centers, hospitals, mailrooms, and apartment complexes.