fig6 u2013 9 it can be seen that the knife selecting arrangement 106 is defined by a selector rotor 108 comprising a cylindrical tube 110 and stub shafts 112 r and 112 l respectively fixed to the opposite ends of the tube 110 the stub shafts 112 r and 112 l are of a smaller diameter than the tube 110 and are respectively received in clearance slots 114 r and 114 l provided in upper edges of the side walls 14 r and 14 l at locations spaced just to the rear of the angle member 20 referring also to fig8 and 9 it can be seen that four different tab arrangements 116 118 120 and 122 are respectively fixed to the tube 110 in longitudinal rows spaced equally from each other about the longitudinal axis of the tube 110 the tab arrangement 116 is designed for blocking all of the spring mounting rods 46 in the retracted position as shown in fig6 so that none of the knives 26 are operative for cutting crop the tab arrangement includes a pair of tabs 124 respectively located at opposite extremes of the arrangement 116 and located between the tabs 124 and cooperating with the tabs 124 and with each.