Master Lock Universal Trailer Coupler Lock. It is designed to fit most 1-7/8", 48 mm, 2", 51 mm & 2-5/16", 59 mm, couplers. It has an advanced locking mechanism that provides resistance to picking & prying, guarding against trailer tow away theft. It is easy to install & remove and resists rust & corrosion. It comes with a limited lifetime warranty.