Daymark

A daymark or a day marker is the daytime identifier (attached signboard) of an aid to navigation (ATON) or day beacon.
Generally, the daymark conveys to the mariner during daylight hours the same significance as does the aid's light or reflector at night.
Examples.
In 1864, the Dart Harbour Commissioners erected a daymark at Froward Point near Kingswear in Devon. Often mistaken for a folly, it is high, octagonal, hollow, tapers gradually from the base to the top, has no roof and no staircase. The base has eight high pointed arches.