is divided by a middle web 19 extending in parallel to the longitudinal sides 18 a into two halves which are essentially mirror symmetrical in relation to this to form the strips 15 a 15 b fig2 each half of the basic body 18 is provided from its broad side that is the upper broad side in fig4 through 6 with a plurality of first depressions 20 whose lower limitations or bottoms are indicated by broken lines in fig6 and 7 the depressions 20 extend into a first plane 21 of the basic body 18 and are arranged at right angles to the longitudinal sides 18 a and to the middle web 19 on the one hand and in parallel to one another on the other hand the number and size of these depressions 20 correspond to the number and size of the strips 15 a and 15 b to be prepared webs 22 left in place between the depressions 20 are provided according to fig5 and 6 at their ends adjoining the longitudinal sides 18 a and the middle web 19 with second depressions 23 which likewise extend into the first plane 21 and are thus connected to the first depressions 20 in one exemplary embodiment which is considered to be the best so far and is shown in fig4 through 7 the height of the parts 22 a of the webs 22 fig6 that remain after the second depressions 23 have been formed and face the longitudinal sides 18 a is reduced so that these parts reach only a second plane 24 which has a distance that corresponds to the thickness of the strips 15 a and 15 b to be prepared.