Ortlieb says about the Gravel-Pack QL2.1 25 L
A pair of universal, waterproof bicycle bags with roll closure: can be mounted on lowrider or rear rack: Quick-Lock2.1 system for luggage racks with up to 16mm in tubing diameter: additional lower securing hook: abrasion-resistant, PU-coated nylon fabric: reflectors.
Accessories available separately: Carrying System Bike Pannier: Carrying strap (E172), QL2.1 hook 18 mm (E211) & 20 mm (E193).