The Aprilaire 6710 is a round slip-in style damper with the following diameter:
The Aprilaire 6710 - HVAC Vent Damper - 10" is a simple solution to providing comfort control zoning in your home. This Aprilaire damper measures 12.6 inches x 3.8 inches x 13.3 inches. This HVAC damper is an automatic motorized damper.
The Aprilaire 6710 specifications: