Abstract: A lock body through bore receives a steel bolt with an array of annular grooves. A locking mechanism secured in the bore has an annular array of locking jaws normally biased engaged with an inserted bolt groove. A jaw spreader has a conical surface for spreading the jaws radially outwardly to disengage the bolt. A locking member normally is radially outwardly of the jaws to preclude such spreading. A spring with a high spring constant, e.g., 300 pound load, axially biases the locking member in the jaw locking position. The spreader and locking member define a locking jaw conical path for radially outwardly releasing the jaws during bolt insertion and for locking the jaws in the opposite bolt withdrawal direction.