It is marked on this old map: http://www.cartesius.be/arcgis/home/webmap/viewer.html?url=http://www.ngi.be/tiles/arcgis/rest/services/seamless_carto__default__3857__1100/MapServer&lang=en together with more, all on the border of La Reid, Theux, Aywaille, so I'm pretty certain it is an old border marker. Added it.