ZM-IR-402 Round Cast Iron Grille 4" Hole Size Without Damper; 6" Exterior Diameter
Made to fit into a 4 1/8" round hole or duct, these grates have an exterior diameter of approximately 2" greater than their boot size (6 1/4" overall diameter). The flange measures 1/8", and is pictured in the side view photo. You may need to modify for different mounting applications.
None better anywhere at any price.