The Difference Between a Heavy Duty Lamp Harp and a Light Duty Lamp Harp is the Thickness of the Metal Wire it is Made From.
Since the Wire is Thicker the Harp Bottom (Saddle) must have Larger Openings to Accommodate the Larger Wire.
Heavy Duty Lamp Harp Approx. O.D. = 0.195"
Light Duty Lamp Harp Approx. O.D. = 0.18"