Abstract: A guide base has at its corners, bolted on transition pieces, each having an outwardly projecting nose with a downwardly sloping bottom surface. A guide post connectable to the guide base has a cavity of a shape corresponding to the shape of the nose. During installation of the guide post, the guide post is suspended on a guide wire to the vicinity of the guide base and is then moved laterally toward the base to cause the nose to enter the cavity. A surface of the cavity cooperates with the nose to cause the guide post to be pulled laterally into firm engagement against the transition piece when the guide wire is pulled upwardly. A locking member can be inserted through the so mounted guide post to lock the post to the transition piece.