That's a set if the non-adjustable Rok Straps. Great product and the perfect companion to your rear extension rack. The non-adjustable version comes in handy when used to strap down a bag or load that is a predictable or consistent size.
I always carry that OR compression sack you see in the photo. I keep my sleeping bag, Therm-a-rest and clothes in there. With the combination of your rack and the Rok Straps, it's quick and easy to snap on or off, and it's very secure.