Those were added a few years after I worked at the bike shop. There were some after market ones that people could buy as an extra that functioned the same way, but a lot of customers did not buy them when they bought the racks. I sometimes would use a short piece of wire to try to prevent the rack from sliding down, hung the wire from a seatpost clamp bolt or a centerpull brake cable hanger, but they did not look very good cosmetically, so store management frowned upon that.