Postal Products Unlimited's Indoor 4B+ horizontal mailboxes are U.S.P.S. approved for replacement and/or retrofit installations ONLY. For new construction, STD-4C units are required. Postal Products Unlimited manufactures the 4B+ mailboxes to USPS-STD-4B+ Specifications. 4B+ mailboxes are ideal for older apartments, condominiums, commercial buildings and residential renovations. Made entirely of aluminum, these units feature a durable anodized finish available in aluminum. The standard "A" size doors measure 6-1/2" W x 5-1/4" H, while the larger "B" size doors measure 13" W x 5-1/4" H. Modules with standard "A" size doors are available in 3, 4 or 5 doors wide and 7 doors high. Units with larger "B" size doors are available in 2 doors wide and 7 doors high. All tenant doors a 5 pin cylinder cam lock with 2 keys. One mailbox opening is required for a master postal lock (installed by the U.S.P.S.) and cannot be used for mail distribution. The front loading door panel swings on a continuous hinge. These units are IN-WALL mount. Postal Products Unlimited is an ISO 9001:2015 certified company. Options: Custom engraved placards, Replacement locks, Additional keys, and Key blanks