Newhaven Harbour

At no.4 Pier Place, opposite the harbour, is this strange pillarbox-shape let into the wall. It has what looks like a wooden door and is inscribed round the top "ERECTED 1775 RE-ERECTED 1900" and round the base "SOCIETY OF FREE FISHERMEN" (difficult to make out, even in large).

The Society Of Free Fishermen, dating from at least 1572, was one of the oldest friendly societies in Scotland, taking care of members and their families in time of need. It ceased activities in 1989.

I can imagine it being a water tap, or maybe a collecting box, or internal mail box: does anyone know what this was really for?