This steel adapter has a female 16x1.5mm inverted flare (24° DIN convex seat) on one end and a 6AN male (9/16-18, 37° convex) on the other. It is very popular for converting certain Porsche fuel systems to 6AN plumbing.
The two-piece design allows you to hold the adapter while screwing down the female threads. This avoids damage to the sealing surface, but requires the correct 24° DIN concave seat on the male fitting. It cannot be used with a crush washer. Sold individually.