The extension rib (322) evolves from the outer cover rabbet flank (321) and extends over a part of the length of the lower corner cut-out (2952). In the head cover rabbet corner (29) near an upper corner cut-out (2951) a lower corner cut-out (2952) is provided, which is flanked by the extension rib. The extension rib is laterally inwardly displaced in relation to the outer cover rabbet flank. The extension rib is provided with a plateau (2953) next to and/or in the direction of the upper corner cut-out. In the upper corner cut-out in extension of an inner cover rabbet rib an extension rib is provided.
