of the stops 37 and 38 are located outside the travel path of the counterstops 40 and 42 in this case the appropriate stroke element 21 is not transported but stays in its at rest position b the plate 39 is located sidewards over the stroke element 21 and grips into a groove 43 which guides the stroke element relative to segment 25 fig5 is in the upper half a plan view and in the lower half a cross sectional cut through the entrainment means 20 it is illustrated as an embodiment wherein for the support of the entrainment means 20 there is provided an insert 44 of a material having good supporting qualities set in a groove 45 having tapered walls with a mild v form relative to each other in this situation it is only necessary for end 46 of entrainment means 20 whose pegs 29 protrude somewhat on both sides to be set into the groove the lid 31 prevents the entrainment means 20 from falling out in fig7 and 8 there are illustrated embodiments wherein the corresponding item numbers are raised by 100 the difference is that the group of transducers 119 are displaced rearwardly in the stroke direction relative to the entrainment means 120 thus the displacement of the entrainment means 120 is.