The Rack Ladder System Aluminum Mailboxes have 1/4" thick extruded aluminum doors with a durable powder coated aluminum finish. Each door includes a five pin cylinder cam lock with two keys. The mailbox compartments have open backs (rear covers are available and sold separately - please see item #3100) and are constructed of 20 gauge steel. All units have the same outside dimensions. Rack ladder system aluminum mailbox units are designed to be installed between rack ladders (sold separately in option menu above). The rack ladders support five mailbox units to form a column of mailboxes. These units can be accessed by the USPS when used in Rotary Mail Centers (sold separately - please see item #3100). One door unit is selected, but also upgradeable in the option menu above to 2, 4, 8 or 12 door units. Unit Dimensions: 23.5"W x 12.125"H x 15.5"D. Door Sizes: One Door Unit: 22"W x11.5"H - Two Door Unit: 10.75"W x11.5"H - Four Door Unit: 10.75"W x5.5"H - Eight Door Unit: 5.5"W x5.5"H - Twelve Door Unit: 3.5"W x5.5"H