I was just given the same level with all the sight glasses removed and am trying to find out how to reinstall them. I am pretty sure that one set is for horizontal, one for vertical and one for 45degrees. The sister set or the sight glass set on the opposite side of the hole is for when you use the other side of the level. For example if you are measuring a horizontal piece of wood the lower buble will line up. When you flip the level over you will use the other buble in the same hole.