These end caps are for use with the Firecrest 100mm Holder Kit. The solid end caps are used when shooting with just 100x100mm filters installed. The vented end caps are used when shooting with 100x125/150mm graduated filters seated in the second (furthest from lens) filter slot.
Each set contains 2 x End Caps (Solid or Vented). The kit contains 2 x Solid and 2 x Vented.
Alternatively a replacement case is available which comes with 2 x Solid and 2 x Vented caps.