On most Rinkers, the enclosure side panels are made from clear plastic all the way around. On mine, the aft side panels are made from canvas. Is that an option offered by Rinker? If not, I guess it's custom work by the original owner.
I love the canvas panels. In hot weather, it's super-easy to roll them up into a bundle that hangs from the hardtop. The canvas also provides shade from the afternoon sun in our slip. But I have never seen it on other Rinkers.