I'm surprised that the post is still standing. These posts bearing signs like "Police Station", "Parking", and "Public Toilet" are in monochrome photos in one of the pilot books I carry. That one cover the area from Crinan to Canna.
Near this point a BBC presenter on the Scottish programme Landward ran aground while on camera. It happened in 2011 and the clip used to be on YouTube but I can't find it.
Sometime after the Vikings in Norway, indicates direction of safe water.
You will find them all around the coast in various forms, along with stone or wood cairns, marking the safe routes.