This is our 316 Stainless Steel Lock fitting for 3/8" OD 304 or 316 stainless steel tube with 3/8" NPT threads on the other end.
They are a 4-piece fitting (no special tools for assembly) consisting of a back ferrule, front ferrule, nut and a body; the 2 ferrules seal tightly around the tube wall. They are compatible with other more expensive stainless Lok style fittings. Common sizes are listed other sizes available upon request.