This steel adapter has 12x1.5mm male threads with a concave flare on one end and a 6AN male (9/16-18 thread, 37° flare) on the other. The 17mm hex is just large enough for a crush washer on the 12mm side. Plating color may vary from photo.
This adapter fits the fuel inlet port on Weber DCOE and IDA carburetors as well as the outlet side of the Bosch "044" (FP200) fuel pump or the newer Bosch FP200-7 Fuel Pump. All of these applications require the addition of a 12mm Crush Washer (Part No. 3278-M12, sold separately).
Convex vs Concave