I thought I found an oval anode on the port side, (about 1 meter from the stern) that looked unaffected by the seawater; it was completely covered with antifouling, so no surprise there.

But when I cleaned it, I found solid brass ( or bronze) It is attached with 2 bolts, which each hold a small zinc anode, recessed in the brass.

Size is about 15 x 7 cm.

Can anyone tell me what it is ?

Thank you !