Looks like the infamous pizza wheel marks the 3880 is often noted for, i.e. the small metal gears of the paper transport. I don't know if your setting the platen to narrow is the best way if that causes the wheels to press harder into the paper/film. Varies with paper surfaces too, and some think pigment ink is also prone to the marks against a dye ink. Maybe try setting the platen wider too?
There are some torsion lift springs in the roller holding those gears/wheels and I've often thought if I had a good spring gauge I could lessen the tension which might help with those marks. Probably a lot of trial and error to the point where the paper might not be transported if too weak. That spring tension seems to be the only variable as to why some get the marks and some don't with same model and paper/ink setup.
I'm curious if the new Canon 17" desktop Pro printer will eliminate this artifact since it is claimed to use some vacuum in the paper transport. Don't know if the new Epson P800 has alleviated it either.