The closure piece can be positioned in a closure position locating on a sealing seat (3) and an opening position raised from the sealing seat. The sealing edge has at least one straight border, which runs horizontally and forms the underside of the opening (2). The sealing seat and a surface of the closure piece working in conjunction with it are of metallic materials. The material pairing of metals involves different degrees of hardness and the devices for movable location effect at least partly a pivot movement of the closure piece.
