The web face of the profile strip (1) bearing on the vertical face of the tread step is depressed from a bottom rearward part to the top leg giving a depressed depth equal to the thickness of the vertical leg of an L-shaped profile (2) fixed to the old step, or again equal to the thickness of an insertion plate (2a). The strip (1) top leg grasping the front shield (3) from above is a solid quadrant section having a single narrow vertical projection. The projecting rib of the strip (1) snaps home into the shield groove so as to hold the shield without play between the rib and top leg of the profiled strip (1). The web of the strip is pref. grooved lengthways on the rib side.
