A throw-lever releasable mounting system for mounting a device in quick-release relation to a support member has a mounting base having a configuration fitting opposed angulated rail surfaces and having a locator key engaging a positioning slot of the rail. Each mounting base provides for mounting and stabilization of optics mounting rings. A locking platform projects from the mounting base and defines a locking opening having a circular hard metal insert therein that defines a receptacle receiving a resilient member and providing for location of the spline/spindle shaft of a rotatable locking plate. A locking plate of a throw-lever that is rotatable between locking and unlocking positions has angulated and curved cam surfaces for forcibly engaging correspondingly angulated surfaces of the rail to achieve cam energized precision locating and locking engagement with the rail. A non-circular section of a spline/spindle shaft of the throw-lever is receives a drive member in non-rotatable and linearly moveable relation. Resilient members are interposed between the drive member and the hardened insert and prevent free throw-lever movement at the release position thereof.