The FLIP 3.1 55mm Threaded Adapter which will allow you to mount any filter to the Flip3.1 frame or TOP FLIP. This is perfect for those critical topside applications requiring a polarizer or split neutral density filter. The 55mm adapter is backward compatible with the original Flip3 frame via the Top Flip accessory and can be mounted on top or side positions with the Flip3.1. Go Pro not included