This may help for a 3100. The sill closest to the cab has holes along the side that bolt it to the front panel, it has the plates. Next two sills are the same, they have the plates (see correct response in next post). Next is the rear and it is an assembly without the plates as it has welded brackets that save the same purpose. Look closely at these pictures hopefully that will clarify it for you.
https://www.flickr.com/photos/advance-design_parts_co/albums/72157631884205225/with/8130516066/

Last edited by Brad Allen; 08/16/2018 3:33 AM.