vertical 3500 series mailboxes have been approved to the required security upgrades of USPS-STD-4B+ and are ideal for apartments, condominiums, commercial buildings and many other applications. Recessed mounted units fit into a rough opening and are secured with mounting hardware through the back or side panels. Recessed units feature a fully integrated flanged collar which fits over the rough opening. The rough opening for recessed mounted vertical mailboxes is 1-1/2" less than the unit's width, 1-1/4" less than the unit's height and equal to the unit's depth. Custom engraved self-adhesive placards (related item #3567) are available as an option upon request. A 4-3/4'' W x 1/2'' H outgoing mail slot (related item #3569) is also available as an option upon request. It will be located in the far left door of the unit unless otherwise specified. If a mail slot is requested, order unit with one (1) additional compartment than the total number of tenants receiving mail.