OK, you're all wet . Just kidding. First, we've decided that it is laying on its back. Because when you stand it up, with the arms at the top facing you, the pedal at the bottom (worn from shoes, I think) is located just right. And the piece of rope inside clearly went to this pedal. When you pull on the rope, one arm moves away from the other(fixed) arm. The wedge that you see allows that piece of wood to be removed for replacing a strip of leather that goes to a "thumb screw" that allows the distance that the arms move apart to be set from about an 1/8" to almost a full inch. And now the big clue: on the top of the box, there is a lot of tar or pitch residue, I think, and the same stuff is on the inside of the arms. The outsides are clean. So what was being pulled apart? There are, though, empty square nail holes from something (a seat?) on the long sides towards the pedal. Could the box have been used in 2 positions?