This steel adapter has a female DIN M16 (16x1.5mm, 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 European fuel systems (Porsche, Lamborghini) 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 it requires the correct 24° DIN or Millimetrique concave seat on the male fitting. It cannot be used with a crush washer. Sold individually.
Male vs Female, Convex vs Concave