Nailor Models 4121, 4122, 4123, 4124, 4125, and 4126 Fire Rated Return Air Filter Grilles have been designed to complement the Nailor range of fire rated supply diffusers. Manufactured to accommodate a throw away filter by others, the grilles have been tested and classified for use without the requirement of duct work or supplementary support, they simply lay in place. The filter grilles are available in two styles: louvered face or eggcrate face.