Ultradyne USA UD Arca Dynalock Rail, MLOK Compatible, 6.15 Long, 4.2 oz, Aluminum Construction, Anodized Finish, Black UD20081 The UD Arca Dynalock Rail can be installed using the M-LOK fasteners provided or by a qualified gunsmith. By upgrading to the Arca Dynalock Rail" you can enjoy the benefits of both Arca and Dynalock accessories on your firearm. The most popular enhancement is the ability to use the Orbit Ballhead with Dynalock" all other Arca Clamp accessories are also compatible.