I think you are right that it is missing its matching identical mate. The clue to me seems to be the wide straps that appear to be velcro and one facing up and one down. My mind is slow right now but there are other things like that, where you make them all the same way, so they can join with each other, so you don't have just right-hand and just left-hand. So I think the velcro straps joined two identical bags together, and the strap area fit over the rear rack, no top clips for the rack.