mechanism designated generally 32 a headed stop pin 34 mounted to slide lock mechanism 32 as described below passes through an elongated linear clearance slot 36 in the base of support member 28 a clearance slot 38 cut through side plate 22 and the arcuate slot 26 clearance slot 36 is long enough to clear stop pin 34 as it moves in support member 28 and slot 38 is sized larger than slot 36 to provide clearance for stop pin 34 as will be described below referring now to fig3 first pivot pin 20 passes through gooseneck link 18 and through a bushing 40 on side plate 22 where it is retained by a snap ring 42 or similar fastener as seen in fig2 second pivot pin 30 is welded at 44 to the base of support member 28 passes through a sleeved bushing 46 on side plate 22 and is retained by a snap ring 48 referring again to fig3 support member 28 has a generally channel shaped cross section and has defined on the inside walls thereof an inner relatively wide two sided slot 50 and an outer relatively narrow two sided slot 52 the slide lock mechanism 32 as seen in fig5 and 6 includes two similar shaped wedge members 54 received within slot 50 and separated by a flexible strip or ribbon 56 of spring steel or similar material the ribbon 56 fits closely between each wedge member 54 and a side of slot 50 and is riveted at 58 at the ends thereof to support member 28 a spring 60 hooked between wedge members 54 is seated partially within a groove 62 formed in each wedge member 54.