The "reverse" zipper sliders are used with the Aquaguard waterproof zippers.  These are the same sliders that are used on my zipper pouches.  To make the sale of such a small item viable, these are sold in 5 packs.  A quantity of 1 will get you 5 sliders, 2 will be 10 sliders, etc.  

The matching #3 YKK Aquaguard zippers are also available on this site.