struts 18 are attached to the hook assembly 15 by means of a pin 19 the struts 18 are coupled to the arms 5 by means of pin 20 referring now to fig3 the basic operation of this mechanism is illustrated fig3 portrays in line form the salient elements of fig1 and 2 to demonstrate how the device is raised and lowered the linkage is a so called 4 bar type used to translate reciprocating motion into linear movement and the converse thereof those of working skill will recognize that the geometry is not regular but chosen by member length and pin position to give the necessary movement without any substantial rotational component to the restraining member from the description so far the motive force to achieve this raising and lowering has not yet been described and will be set forth herein in fig3 the arm 5 pivots about a point a point a corresponds to the pin 6 the length of the strut 7 the lever assembly 9 and the position of point c the pin 8 is selected so that the path of point e pin 14 is very close to a straight vertical line that point of travel is illustrated by the chain vertical line in fig3 labelled u201c locus of points e u201d the position of points f and g which correspond respectively to pins 19 and 20 respectively are selected so that the hook assembly 15 remains horizontal as the other members rotate thus as the device is raised from an inoperative position shown in fig4 to an operative position illustrated in fig2 point e moves vertically and the hook assembly 15 remains in a horizontal position referring.