I dont think it's anything to do with the plastic casing found on either side of the drum unit.
I ran a C1100 with both covers removed, and still had the smear problem.
I cured it by accident, as two metal rollers from beneath the transfer belt fell out when I was messing around. Thinking that I'd never get it all back together again, I messed about with the metal rods and belt for some time, until it all fitted together again.
When I re-tested it, the smearing problem had gone. I was pretty amazed, but pleased of course.
I think the problem could be excessive friction on the belt, caused by the two white nylon rollers located at each side. They press on the belt edges at a 45 degree angle, presumably to keep the belt central. I took these off in order to get it all back together again, and thats the only thing I could think off that could have an effect. I did refit them, and the smear problem was still absent.
Maybe anyone with this problem could try doing this and letting us know whether it worked.
Remove the 6 screws that hold the top cover on (the one with the handle). Then remove one screw from each side that holds the white nylon angled rollers in place.
Put the cover back on, and test to see if your smearing problem has been resolved.