Because in many cases, the same thread is used on both the light and the heavy series DIN connectors, the adaptor can only be correctly identified by measuring both the thread and the tube size.
Otherwise, the adaptor nut almost always has the size and series stamped on the back face.
BSPP British Standard Pipe Parallel
Country of origin: Britain
Thread angle: 55 degrees (Whitworth)
Seat angle: 30 degrees