This is an Interfit Strobies Flex Mount for the Nikon SB900 & SB910 Flashes. It is an integral part of the Strobies series of flash light shapers.
There are several light shapers that work with the Flex Mount. A Snoot, Barndoor Set, Softbox, Honeycomb Grids, and Bounce Cards are available separately.
The Flex Mount is correctly fitted when your flash cannot be pushed any further inside it and is flush with the central hole.