We're working out what we think - we'll add our view here!
Tread Lite Gear says:
Sold in pairs. Waterproof YKK zips with dual sliders and dyneema pulls. Rear elasticated straps and slik clips each end to anchor on hip belt. You will need some form of anchor point on the hip belt you are attaching to. As with all manufacturers of external pockets Tread Lite Gear recommend moisture sensitive items are protected with an additional ziplock bag.