Abstract: An assembly for mounting on a standard mounting platform, such as a Picatinny-type rail, the assembly provides engaged contact between the rail and the surfaces of a base and a clamp. The clamp is rotatably attached to a base, allowing a user to quickly shift the assembly between a rail-engaging position and a disengaged position. The base also preferably contains a locking mechanism that allows a user to lock the assembly in a rail-engaging position. The assembly is particularly useful for the mounting of accessories, such as scopes and sights, onto Picatinny-type rails that are affixed to firearms.