This is an extended tube nut for 3/16" brake tubing. The outside has 3/8-24 SAE threads all the way to the end, and the inside is contoured to fit a standard SAE 45 degree inverted (double) flare on common 3/16" or 4.75mm steel brake tubing.
The tip has no unthreaded lead portion, so the port must be threaded all the way down (not for use on British ports requiring a "bubble" or convex flare). Extended nuts like these are typically used in high-vibration areas, where the tubing needs a little extra support. High-pressure brass. Sold individually. Tubing not included.