Well, I have always referrred to sill covers or a side skirt as the same thing.......have revised the subject title to avoid confusion
I thought that perhaps there was some plate, to cover the bottom of the door surround, for you to step on, as seen on some other cars.
I do have a set of 'S' side skirts that I bought from Simons (not cheap), but they will go on my car when I get to reconditioning the body work.