DC-11 Twin Hold-On Tyre Valve Connector, Female Thread Rp 1/4, Open End

There are two types of connector. It is therefore important when purchasing connectors to specify whether a closed or open end type is required.
A Closed End connector is used when air is supplied directly from a main air line (i.e. compressor or air receiver). 
A non-return valve unseats on connection to the tyre's valve, allowing air to flow into the tyre. 
The handles on these connectors are red in colour.
An Open End connector is used with Tyre Inflator and gauges. 
This connector has a pin which unseats the tyre's valve when in use. 
The handles on these connectors are black in colour.