Utilize the molded concave design of these Traex® Gray Bus Box Lids, which allow for positive stacking of bus boxes. Stacking makes the boxes more useful and helps conserve space. This bus box lid is designed for use with the Traex® 15" x 20" Gray Bus Box (Item Numbers 667747 and 667751). In compliance with California HB1200, Vollrath has notified us that this product contains PM83356 Gray.