UMAX 50 Universal Trailer Coupler Lock - The UMAX 50 features a rugged corrosion resistant finish and provides instant security for your boat or trailer when they are not hooked up to your vehicle. It features 1/2 turn technology that cannot be false locked. For dual security, add a TRIMAX chain or cable secured to an anchor point! Easy installation simply slide ball up into coupler and replace u-shackle. Fits all of the following couplers and more...1-7/8 inch or 2 inch Fulton and Atwood 2 inch, UFP Bulldog 2-5/16 inch couplers and Fulton Heavy Duty Surge Brake Coupler (Reverse Mount).