The disclosure relates to a fastening device with a stem which is engaged on a first end by a cross-dowel with a transverse threaded aperture and on a second end by a cylinder with a set-screw. The second end of the stem includes an aperture surrounded by an oval-shaped longitudinally elongated countersunk region. This aperture can capture the set-screw within a small range of longitudinal positions, thereby allowing for fine adjustments to be made to align the sidewalls of adjacent post segments joined by the fastening device.