The seat rail mounted compontent of the Spinelock stability system. Sold individually as a spare or replacement. Having a second can be handy if you use your Spinelock seat bag on multiple seats or bikes. A Torx-25 wrench is needed for installation. you likely already have one on your multi-tool! Important information on use and fit: - Not compatible with carbon fiber saddle rails - Only compatible with traditional style saddle rails and is not compatible with most models of Brooks saddles including (B17, B66, B67, Flyer, and similar). - This product is not designed for use with dropper seatposts. - A Torx-25 wrench is needed for installation. You likely already have one on your multi-tool. Materials: Forged aluminum.