The pre-flashed, pre-assembled mounts are manufactured of fiber cement with an ABS flange. Providing a water management solution, blocks are easy-to-install and provide a clean finished look. To be used on electrical boxes. 6" x 8-1/2". No. 3SMR68AW: Surface Dimensions: 6" x 8-1/2", Rectangular Knockout: 2-1/2" x 4.125", Material: Fiber Cement, Color: Artic White No. 3SMR68P: Surface Dimensions: 6" x 8-1/2", Rectangular Knockout: 2-1/2" x 4.125", Material: Fiber Cement, Color: Primed