The 5/16 version of the AN6 quick connect adapter. Manufacturers use 5/16 line widely on return circuits and on the feed side of smaller-capacity engines, so having both this and the 3/8 variant on hand covers most of what you will meet under a modern bonnet. It lets you plug an AN6 hose end directly into the existing quick connect without cutting the factory line, which matters on a car that must be returned to standard or where the original line is expensive to replace. Push the coupling on until the retainer clicks fully home, then pull to confirm it has latched before pressurising the system.