This is an Interfit SGM500 Flex Mount for the Vivitar 285HV on-camera flash. 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, & Bounce Cards are available separately. The Flex Mount is correctly fitted when your flash cannot be pushed any further inside it & is flush with the central hole.