Abstract: A coupling or adapter assembly that serves as a transition device for interconnecting a fluid source, such as a fire hydrant, to a conduit, piping system or hose includes an adapter portion of generally cylindrical configuration having a fluid conveyance bore of non-uniform diameter extending therethrough and annular threads at opposed ends of the adapter portion. The annular threads at one end of the adapter portion are of a stepped threaded configuration in order to engage the inner annular double threaded configuration of a ring member that is externally threaded onto the adapter portion thereby binding the adapter portion to the ring member so that they function as an integral, one piece transition device interconnecting, for example, a fire hydrant to a Storz-type coupling of a hose or conduit.