1/4 MPT X 5/16 Barb (Inlet for Secondary Low Pressure Regulators ONLY)
This fitting will screw into the side of a secondary regulator.  It has a 5/16" hose barb on it for attaching CO2 Gas hose.
When using a secondary regulator with a barb side, you must have a primary regulator attached to your CO2 tank, then you can have your CO2 gas hose attach from your primary regulator to your secondary regulator.