metal strip material in this case the edge 17 lies at the free end of the channel 6 from that point the formed material first follows the inner curvature of the channel 6 and then passes over to the outer layer of the slat 5 as a result of the forming process in the lower region of the slat 5 the metal strip produces the outer layer of the slanting first front wall segment 8 and then passes into the outer layer of the vertical second front wall segment 9 additionally the metal strip adjoins the edge 11 in the outer layer of the bottom surface 10 from there it extends to the slot edge 12 at the slot edge 12 the material is bent over onto itself and extends as the inner layer of the bottom surface 10 up to the edge 11 here the material rises vertically upward and it lies closely against the outer layer of the second front wall segment 9 the sheet metal material then continues as the inner layer of the slanting first front wall segment 8 lying directly against its back or inner side as far as the root of the slat 5 here the formed metal strip provides a partly cylindrical curved surface 18 as the inner layer of the bend piece 13 adjoining this the formed metal strip provides an interior layer of the rear wall 14 and thus up to the slot edge 15 here the.