A hinge mechanism comprises first and second chock plates and an adjustable shaft. Each of the chock plates includes an aperture in the form of a figure of eight, the first and second circular portions of which are similar and are connected by a neck portion of a width less than the diameter of the circular portions. The shaft is longitudinally adjustable between a first retracted position in which the sections of the shaft aligned with each of the chock plates have a cross-section which corresponds to the circular portion of the aperture and a second extended position in which the sections of the shaft aligned with each of the chock plates have a cross-section of a diameter which is less than the width of neck portion.