A set consists of two FCS II side plugs. Buy this set together with a center plug to create a complete thruster set. If you want a quad setup buy a total of two sets. Most often the quad rear fin boxes have less of an angle than the front fins. So for example one 9 degrees set for the fronts and one 5 degrees set for the rears.
Small color accents might be used to indicate the difference between 0, 3, 5, and 9 degrees. These colors are often not visible on the product photos. All plugs come WITHOUT screws.
FCS II Fin Box installation instructions here.