A separate block can be noted running along the bottom of the relief. This suggests that there were either not enough blocks on site of sufficient dimensions to fill the space required for the relief or that it was not felt important to have blocks of the same height as the relief.
Again, as so often on the arch, the holes made by the removal of metal clamps can be seen at the top and bottom of this scene. The number of clamps used on the arch is quite striking. There must, of course, always have been a large number of blacksmiths on site for sharpening tools but these individuals would also have been responsible for creating and fitting these clamps and probably also pouring in the lead that coated and filled the void around them.