This is the small square anode on the side of the gearbox casing.
For Mercury Mariner 4, 5, 6HP 4-Stroke outboards. Also used as a cylinder block anode on Mercury/ Mariner 25hp and 30hp EFI models.
Gearcase side anode.
Square gearcase anode for F8 and 9.9 four-stroke Mariner and Mercury outboards and other outboards listed below.