Anyone know what this is? We have two of them. The long notched piece moves up and down. Thanks!
It's a ladder jack. The square brackets go on the legs of an extension ladder and the hook goes over a higher rung. You can hang a staging plank between two ladders in the resulting triangle, which is why you have a pair of them.
The drawing below is backwards, but representative of the general idea. The jacks would normally be hung under the ladder so the plank is close to the building.