I am not positive, but it appears the guide line is actually a string, or small diameter cord. Possibly stretched between permanent pins placed at each end. The "box" appears to be a "sifter" so the hammer blows shake the chalk dust out. The men are setting it down very carefully, perhaps to avoid shaking out unwanted dust to make a phantom line. I can see where doing it this way might be slow, but also make the line quickly and easily to repaired if it gets "erased" during a game.