I have never seen a brass freeze plug with, what appears to be, an aluminum wafer glued or pressed on to it. I have included photo, but flash has bleached out the aluminum (size of a nickel) part. Motors are anti-freeze cooled, so saltwater should never be able to eat out. These are V8 Chevy motors. Does anyone have any idea what or why plug manufacture would add this aluminum to their brass plugs? THX!