ZM-IR-601 Round Cast Iron Grille 6" Boot Size With Damper; 7 1/2" Exterior Diameter
Made to fit into a 6" round hole or duct, these grates have an exterior diameter of approximately 1" greater than their boot size. The damper is made from finely cast iron with a Lost Wax Cast brass operator. None better anywhere at any price.
You may need to modify for different mounting applications.
Some resistance may be felt when opening and closing the cast iron louvers for the damper. The friction helps to keep the louvers in a set position to your liking, and withstand normal increases in air pressure caused by higher fan speeds from heating and cooling units.
For additional measurements see ZM-IR-601_cutsheet.pdf