Zinc anode. Replaces Mercury/Mariner 31640, A1, Q4. For use on Mercruiser R/MR/Alpha One (sterndrive & transom assy 1983-90 & prior drives. Use on Mercury/Mariner inline 3,4 & 6 cyl. V6 outboards. Also replaces Honda 41107-ZV5-00. For use on Honda extension cases BF25, BF30, BF35, BF40, BF50 (1997-up)