These covers have ribs that are about 1/8" wide and are smooth between (see close-up photo). This listing is for your choice of covers:
Leviton Large Hole (1-3/8") Cover I have one sealed with screws, the other is open, no screws
Leviton Blank (no holes) Cover No screws
Leviton Small hole (5/16" diameter) I have one sealed with screws, the other is open, no screws
Leviton Medium Hole (5/8" diameter) Cover In wrapping, includes screws