However, another retainer serving the same functions as slots 18 a, 18 b may be substituted therefor for one of the two of left and right slots 18 a, 18 b, including structures so formed to retain grate-ends 4 a, 4 b (shown in FIG. 3A) from upward movement and movement away from the opposite retainer, and to permit rotation of grate-ends 4 a, 4 b about transverse axis 7.