This aluminum fitting has 9/16-24 (6AN) male threads on one end and a male 4AN fitting on the other end. The 6AN side does not have a sealing flare and must be sealed at the face of the port with a crush washer. A soft copper crush washer is included, but if you prefer you could use an Aluminum Crush Washer (Part No. 3278-06, sold separately in packs of 5).
One of the applications for this fitting is on the inlet port of the Tilton 77-Series Rear Pivoting Master Cylinders.