Advantageously, inproducing the housing cap 7, for example the method for manufacturing a metal cap usable as housing cap 7 described in the already mentioned WO-A 03/021202 can be used, in which such is formed by the welding of two cap halves of essentially identicalform, especially such cut out of a plate-shaped stock, with an edge bead, especially a bead in the form of a quarter torus.