This steel adapter has a female DIN M16 (16x1.5mm, convex Globeseal seat) on one end and an 8AN male (3/4-16, 37° convex) on the other. It is very popular for converting certain European fuel systems (Porsche, Lamborghini) to 8AN 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 a DIN 7631 60° or DIN 3902 / Millimetrique 24° concave seat on the male fitting (the Globeseal seat is not designed to work with Japanese JIS 30° seat). It cannot be used with a crush washer. Sold individually.
Male vs Female, Convex vs Concave