USPS Domestic Mail Manual, Section 508.3.2.3 - Locked Box - a mail box with a lock must have a slot that is large enough to accommodate the customer's daily mail volume. The parties further agree that there may be instances during a mail count when additional time is warranted for making more than one pass to a compliant locked mail box dependent on the particular fact circumstances. Step 4 ( L-24 ), in part: the parties agree that according to Postal Service standards, a locked box should have a slot large enough to accommodate the customer's normal daily volume of mail. Is theft from mail boxes a problem in your area?
By chance is this 'slotted' mail box a locked mail box? Hangingbythreads - 'Does anyone have the rule that states notices for parcels can be left for parcels that would fit in a regular mailbox but can’t because it’s one of those slot boxes? '