12 and 14 are made with laterally projecting flanges 21 and 22, respectively, which are suitably holed as at 28 to receive bolts or other securing means; and to provide for the pivotal support, of the wing plates 10 and 11, the flange 22 is made with forward extensions comprising upper and lower hinge ears 24: and 25 carrying two hinge pins 26, of which there is one for each wing plate.