20 is provided with an annular boss 30 having a pair of substantially diametrically opposed peripheral protuberances 30a which independently coact with the lug 20d formed on the bracket base portion to limit the pivotal movement of the link member in a given direction either towards or away from position ii in order to retain the link member 28 in either position i or ii a washer assembly 31 is provided which encompasses the threaded shank portion 26d of the fastening device 26 and is interposed the knob k and a recessed surface portion 28b of the end 28a of link member 28 the washer assembly 31 in the illustrated embodiment includes a resilient washer 31 and a rigid washer 38 having a peripheral flange 33a which encompasses the annular periphery of washer 32 the projection of the flange 33a is less than the thickness of resilient washer 32 so that as the knob k is threaded onto shank portion 26d friction will increase between washer 32 and the recessed portion 28b of the link member 28 thereby providing greater resistance to pivoting of the link member 28 relative to the bracket base portion 20 as seen in fig3 the.