Abstract: A lock is adapted for use within a box, container or fastening wherein the lock makes the box, container or fastening freely openable for a first opening, but locked after once opened and reclosed. The lock shown in a preferred embodiment is adapted to fit within a standard rural mailbox of the type approved for use by the U.S. Postal Service wherein an inner lock box is formed within the standard mailbox by a front hinged plate operable in conjunction with movement of the front door of the mailbox and a curved top releaseable by complete closure of the mailbox door wherein such inner lock box front plate and top remain in the completely spread and open position when the mailbox is first opened (presumably by the mailman) and wherein the inner lock box front plate and top are in the closed and locked position upon the second opening of the mailbox such that a key is required to re-open the inner lock box.