From Manufacturer
This FotodioX WonderPana FreeArc 66 Essentials ND16 & GND 0.9HE Kit features a FreeArc Core filter holder dedicated to the Sigma 14-24mm f/2.8 DG DN Art lens with either a Leica L or Sony E mount. The all-metal FreeArc Core clamps onto the hood of the lens at the turn of a rotating ring. The kit also includes a 145mm ND16 filter, which screws onto the threading on the front of the FreeArc. The filter is threaded on both sides and can be stacked with other 145mm filters from FotodioX, such as with additional ND filters, to achieve a higher ND factor.
Additionally, the kit includes the 6.6" Holder Bracket, a two-part bracket that's also referred to as "the wings". Each "wing" attaches to a side of the FreeArc Core. With both secured to the FreeArc, the wings allow you to mount the included graduated ND rectangular filter. Moreover, the wings fit two rectangular filters at the same time if the 145mm circular filter isn't screwed onto the FreeArc. When a circular filter is mounted, the wings only fit one rectangular filter.