and the two channel legs 54 58 of which projecting from a base plate 52 have a free height k of about 3 5 mm is sketched in fig5 the left hand channel leg 54 therein forms a side face 53 at right angles to the base plate 52 of the channel profile 50 and has an inner contour 53 a which is in the shape of a circle segment and forms an inner rib 56 with an end region 55 likewise in the shape of a circle segment of the channel leg 54 the other channel leg 58 adjoins said base surface 52 by means of an outer inclined surface 57 and likewise has toward the channel space 62 an inner contour 57 a which is in the shape of a circle segment and which likewise merges into an inner rib 64 with the inner contours 53 a and 57 a in the shape of a circle segment which respectively adjoin them the inner ribs 56 64 define an undercut region 63 of the channel space 62 fig6 in cross section the abovementioned inclined surface 57 adjoins at one end the outer surface 51 of said base plate 52 and at the other end an end inclined surface 59 which encloses with the outer inclined surface 57 an angle w of about 135 u00b0 and merges into an end region 55 a in the shape of a circle segment at a distance from the channel profile 50 an insert profile 70 of virtually rectangular cross section is assigned to said.