used as stand-alone mailbox units or in multi-module layouts. Larger compartment size accommodates catalogs, magazines and other types of "flat" mail without folding and rolling, thereby minimizing potential damage of the mail. The STD-4C specification also provides for both a front-loading as well as rear-loading mailbox module. Rear-loading 4C mailbox systems must be installed in a secured room. If mail is delivered to each mailbox individually by the USPS, a key must be provided to the USPS to gain access to the secure mailroom itself (does not apply to private delivery where USPS delivers to one location and mail is distributed by others.) It is generally preferred to have a key keeper installed outside the secure room. This way, the USPS can install their normal Arrow lock in the key keeper to gain access to the key for the mailroom. Approved manufacturers Manufacturers must produce a mailbox which meets or exceeds all USPS Specifications and then submit a prototype to the USPS for rigorous testing and approval. All STD-4C compliant mailboxes must meet the USPS design and installation regulation in order to receive a "USPS Approved" designation. STD-4C 2BGlobal Florence Manufacturing Company Jensen Mailboxes Postal Products Unlimited Salsbury Industries Security Manufacturing References United States Postal Service