Ladder bracket on the back of a ladder. These are made in various styles. Some fasten to rungs. This one attaches to the siderails. It is not, therefore, likely to depend upon a defective rung and so it is safer. They are used as shown on the outside of the ladder to hold a plank at the top of two extension ladders in painting roof cornices. It is possible to swing these brackets to a position on the under side of the ladders. There they make an excellent scaffold from which to work on a wall or window frame.
International Association of Master House Painters and Decorators of the United States and Canada Painting and Decorating, Working Methods33