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. You can use all your existing FCS fins with the FCSII system. The FCSII plugs are backwards compatible - grub screws can be used within the FCSII plugs to secure your existing dual tab FCS fins. FCS has designed a set of unobtrusive silicone infills to fill the gap created by the old fins in the new system. The 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. The infills replicate the flush leading edge of the new fin design improving the performance of the old fins.