Boxes containing banknotes are secured by a mechanism utilizing a latch plate that can be removed and replaced automatically by a device of which the parts include a top slide equipped with a pivoting arm and a pusher designed to hold the latch plate steady and in tension from two points of contact; the device is also equipped with a bottom slide carrying a gripper of which the two jaws operate in concert one with another and with the pusher and the arm to release the latch plate from the position in which it secures the box.