Martyr zinc anode designed to protect gearcase from corrosion salt water. Mercury Cutdown Skeg Zinc | Martyr CM46399Z Mercruiser trim tab anode Zinc anode for salt water applications Short trim tab provides extra clearance for high rake propellers Zinc replacement anode for Aluminum Mercury anode p/n 822777Q1 For a magnesium replacement see mercury p/n 97-822777A2