Ortlieb says about the Vario PS QL2.1
UPDATED:
Backpack with roll closure and Quick-Lock mounting system: can be used as a bike bag or backpack: quick, intuitive conversion between functions using a reversible front pocket: outside pocket integrated in the flap (not waterproof!): side mesh pocket: inside pocket with laptop compartment: handle: chest strap: reflectors.
QL2.1: Quick-Lock2.1 system for racks with up to 16mm in tubing diameter:
Accessories available separately: Anti-Theft-Device (E124): QL2.1 hook 18 mm (E211) & 20 mm (E193).