round cavity 40 followed in the heel 38 itself by a threaded cavity 41 as visible in fig4 the flat head 33 39 of the screw 33 is circular and comprises a straight slot 42 internally longitudinally curved not shown enabling to introduce in it a circumferential segment of a flat round piece for example a coin which everbody possesses at all times furthermore as visible in fig3 the flat head 33 39 has a square cut edge 33 34 in order to enable it to sink into the cut out 32 of the sole 31 the latter is equally round and with circumferential dimensions just greater than those of the head of the screw 33 but only the upper part 32 39 of the cut out 32 over a sufficient depth is nearly of the same diameter as the head 33 39 of the screw whereas the lower part 32 34 has a smaller diameter so that the screw 33 is stopped at the bottom of the cut out 32 the cut out 35 of the reinforcing plate 34 which is intended to receive the ferrule 39 of the heel 38 will have therefore a peripheral dimension slightly larger than the said ferrule 39 the shape of the cut out 35 as of the ferrule 39 can be curved or even circular over nearly all the length of its periphery but with at least a cut off corner respectively 35 39 fig6 in the cut.