41 is positively fixed in central recess 39 by fastening device 40 cap 41 has an outward side 42 which is convexly curved in conformity with outward side 38 of cover disk 33 so that a flow optimized rotary body is produced fastening device 40 preferably consists of nose like extensions 43 on cap 41 as well as circumferential groove 44 in recess 39 moreover cap 41 has a central bore 48 which makes it possible to insert an appropriate tool so that cap 41 can be readily dismounted if necessary an indicating groove 49 see fig3 on cap 41 makes the particular operating state of opening cylinder recognizable in an unambiguous manner cover disk 33 whose diameter is distinctly greater than the diameter of annulus 30 has graduated sealing steps 45 formed annularly in the area of its outer circumference which mate with correspondingly formed graduated sealing steps 46 on opening cylinder carrier 47 of opening cylinder housing 17 the relatively narrow axial circulation slots of this stepped sealing arrangement 45 46 result in a minimizing of the entry of infiltrated air into opening cylinder housing 17 even though the operating state of opening cylinder 21 is already rendered recognizable by the indicating groove 49 in an unambiguous manner rotatable cover disk 33 is also covered for safety reasons by shield 50 arranged at a spacing relative to cover disk 33 this shield 50 is mounted on cover element 8 in such a manner.