indicated at 62 and is shown in sectional detail in fig5 and 7 this locking means includes a block 64 carried on the inner face 66 of web 44 of the generally channel shaped bracket arm 40 block 64 is bored and internally threaded at 68 and has a smaller diameter counter bore at 70 a set screw 72 is received within the threaded bore 68 access to the wrench flats 74 of the set screw 72 is through counter bore 70 the face 76 of side bracket 28 bears denting means 78 which can comprise a surface recession as shown in fig5 that is coaxial with threaded bore 68 to receive the end of set screw 72 when the latter is advanced from its threaded reception in block 64 in the preferred embodiment detent means are provided whereby the bracket arm 40 can be locked in raised and depressed positions fig6 illustrates such detenting recessions 78 and 80 which are provided on face 76 of side bracket 28 on a common radial spacing from pivot pin 48 carried by the side bracket referring now to fig7 the set screw 72 is shown in its advanced position from its reception in threaded bore 68 of block 64 such that the end of the set screw is received within the detenting recess 78 thereby locking bracket arm 40 against pivotal movement on side bracket 28 the cover bar 52 that overlies the distal portion 54 of the crash bar 20 intercepts the hook arm 36 of a wire such as wire 32 described with regard to fig1 the attempted actuation of the.