Abstract: The lower profile element is substantially of a U-shaped section and includes wings having each a median zone with a semi-cylindrical channel prolongated by a bent-over portion provided for covering V-bent vertical ends of side branches of the upper profile element. A wing of the lower profile element is cut out in order to form a row of teeth having each a trapezoidal shape prolongated by a rectangular shape in order to cooperate with an L-section latch-comb formed with rectangular openings and freely mounted on a pin which is displacable by a control of the slide. A spring applies to the latch-comb a pressure permitting a passage of teeth of the latch-comb through openings formed in the upper profile element so as to cause a penetration of the latch-comb into the openings between the row of teeth of the lower profile element for latching the upper profile element on the lower profile element.