To fill the gap between the front tab of the original (dual tab) FCS fin and the front of the FCS II plug front slot.
 
Overview
FCS II Compatibility Infills are designed to fill the gap between the front tab of the original (dual tab) FCS fin and the front of the FCSII plug front slot. These infills are made from high quality silicone rubber which will not damage the plugs or fins (they are also extremely durable, light and built to withstand extreme temperature and corrosive conditions).
Centre Compatibly Infill (x1)
Side Compatibility Infills (x4)
Grub Screws (x10)
FCS Key (x1) 
Reusable zip lock bag