Magnesium Anode Fits Mercruiser Alpha 1 Gen 1 and Most Mercury Outboard Motors 35HP and Above. Replaces Mercury 31640M, Martyr CM31640M, Sierra 18-6011M, 41107-ZW1-003M, Tecnoseal 00800MG. This Is A US Military Grade Magnesium Anode Made To The Highest Quality. Magnesium Is For Freshwater Use. This IS A US Military Grade Magnesium Anode Made To The Highest Quality. Magnesium Is For Freshwater Use.