These fittings are designed to work with our stainless steel hard line. They are a double ferrule design and come supplied with assembly instructions (also available here).
We offer these fittings with an AN6, and AN8 male thread to suit the 3/8" (10mm/AN6) tube, and for the 5/16" (8mm) we keep a fitting that has a 1/4" BSPP female thread. This means with the addition of a simple adaptor, you can connect to multiple hoses sizes, from AN3 to AN8.
We do not recommend using any other style of hard line adaptor with our stainless steel tube, as they often use a 'soft metal' olive, and don't have the capability to form a complete compression connection.