A first coupling member has a peripheral facing surface with a pair of annular slots, each of a different length and width. A second coupling member has a lock ring longitudinally slidable thereon with a facing surface having a pair of annular raised locking tabs sized to correspond to the respective annular slots. A resilient annular spring mounted within the lock ring urges it toward the first coupling member so that the locking tabs engage their respective slots when the coupling is tightened to a predetermined extent during rotation in the locking direction to releasably and positively lock the coupling. An indicator stripe on the peripheral edge of the first coupling member is exposed before the locking tabs and slots are engaged. Alignment between the respective locking tabs and slots allows longitudinal movement of the lock ring so that it covers the indicator stripe. One of the coupling members has a peripheral groove and two angularly spaced longitudinal keyways of differing widths which intersect the peripheral groove. The annular spring has an internal peripheral edge with a plurality of peripherally spaced spring tabs for holding it in the peripheral groove. A spring key on the internal peripheral edge of the annular spring has a width corresponding to the width of one of the keyways to rotationally position the annular spring with the coupling member. The annular wall of the lock ring has two angularly spaced keys of differing widths which correspond to the widths of the keyways and are received therein to angularly position the lock ring with the coupling member and the spring.