It is a multi-function Arri Rosette(M6, 31.8mm diameter) Mount.
There are two 1/4''-20 threaded holes and two #8-32 threaded holes on this rosette mount. When used with the Zacuto VCT plate, it can be attached to both sides (with the 1/4''-20 threaded holes) and the front area (with the #8-32 threaded holes). Where to mount the rosette is based on your needs.
Material(s): Aluminum Alloy, Stainless Steel