20 having a pair of apertures 21 formed therethrough to receive the same screws 17 used to secure the lip attachment 15 to the coupling section 10 or somewhat longer screws if necessary the mounting plate 20 is installed on top of the lip attachment 15 fig1 and a notch 22 in the mounting plate 20 between the apertures 21 straddles the raised part 18 of the lip attachment 15 the rigid body 19 further comprises a yoke portion 23 perpendicular to the mounting plate 20 and extending above and below the mounting plate at its top and bottom the yoke portion has short horizontal arms 24 having coaxial apertures 25 formed therethrough the yoke portion 23 is also fixed at an angle to the longitudinal axis of the mounting plate 20 as shown in fig1 when the rigid body 19 is installed on the coupling section 10 in the described manner through the two screws 17 the yoke portion 23 is disposed near corresponding ends of the two coupling sections 10 and 11 with the yoke arms 24 near the top and bottom thereof a rigid blocking pin 26 is then inserted through the aligned apertures 25 with its head 27 resting on the upper horizontal arms 24 the blocking pin extending vertically across the two coupling sections 10 and 11 in this position the rigid blocking pin 26 positively prevents the flange 14 of the upper coupling section 11 from swinging outwardly in its swiveled relationship to the lower coupling section 10 in the direction which could result in separation of the two coupling sections in truck application the upper coupling section 11.