Best quality QD on the market. Locking feature is a huge plus. PN: QD-YF-S Description:  Female QD: 9/16″-18 Male (Connects to a standard regulator hose) Male QD: Connects to any standard 9/16″-18 2nd stage regulator fitting. Details: Female QD has lock built into sleeve.  Once connected, rotate the sleeve counter-clockwise 90deg to engage the lock. Male attaches to any 9/16″-18 male fitting or standard thread 2nd stage regulator. Solid Brass Construction Made with 02 compatible material