the tackle element which bar shaped part is provided over its full width with a symmetrical t shaped recess which opens out with the upright part of the t shape into the flat side which forms at right angles to the lengthwise direction of the tackle element the ending flat wall of said tackle element the parts 28 39 and 29 39 are the same shape but in the position in which they have to be assembled one of them is turned through an angle of 180 u00b0 relative to the other about an axis parallel to the longitudinal axis of the flanks 28 29 if we look at the part 28 39 of a down lying left flank 28 see fig3 b it comprises a horizontal bar shaped part 35 with a bottom flat side in line with the bottom side of the element 28 34 laterally running out wider over a width corresponding to the width of the assembled tackle element and with a height which is less than half the height of the element 28 34 a u shaped channel 36 is provided over the full width in the top side of this bar shaped part said channel 36 has a front flank which slants down backwards widening the channel 36 towards the bottom over the width of the element 28 34 the rear flank 37 of said channel is made higher and via a horizontal part 38 which connects along the top over its full width to said higher wall 37 and a vertical wall 39 connecting over.