Goodyguy is right, it lays out 16” centers for studs on the top and bottom plate. Starting at the wall the wheel rides the plate and each time 0 comes around that’s 16” and you mark it (hence the name “Mark 16”). I’ve heard of one of these used but have not seen one. They had a tendency to slide if the wheel got a little stiff giving an erroneous measurement. No idea of value.