which in turn are rigidly connected to the chassis 46 of a truck tractor 48 the pivotal mounting of body 1 to brackets 4 can be with a single shaft 5 extending through both pivots or alternatively a pair of pins or shafts 5 which extend through bearings 5a fig1 on the fifth wheeh coupler plate body 1 both of these modes being conventional at the inner closed end of slot 3 is a head or jaw element 7 having a semi circular recess 7a facing rearwardly a shaft or bar 8 of generally cylindrical shape or the like projects forwardly from element 7 through the cross leg of a u shaped guide bracket 50 and also slidably through a cross rib 53 depending beneath body 1 element 7 and its shaft 8 are shiftable toward the front of the coupler body against the bias of a compression spring 9 coiled around shaft 8 and extending between the cross leg of element 50 and flange 53 forward movement of element 7 causes the second jaw or holding lever 11 to pivot about its pivot pin 12 into engagement with element 7 to retain a kingpin 52 between the jaws a locking lever 58 is pivotally shiftable to lock element 11 in latched condition element 11 preferably is provided with a safety stop or stop block 54 pivotally mounted on cross pin 56 in the mouth of slot 3 this detailed structure of the fifth wheel coupler subassembly 42 is conventional the novel anti jackknife subassembly 44 cooperates with this structure and particularly with the forwardly shiftable shaft 8 connected.