This stainless steel adapter has a male 12x1.5mm inverted flare (concave seat) fitting on one end and a 3AN male (3/8-24, 37° flare) on the other. The 9/16" hex is large enough to support a 3/8" crush washer (sold separately) on the 3AN side, but not on the 12mm side. Sold individually.
Convex vs Concave