The serrated portions of the studs are fitted in those portions of the mounting holes 14 close to the other side face of the mounting flange 13b. A groove, which may be wedge shaped, is formed in the bottom surface of the protective layer. The studs are particularly geometrically configured and have shoulders that are larger than the stud barrel and have heads that are larger than the shoulders which results in a mechanical locking between the slots and the studs upon rotation of the cell with respect to the stationary studs.