Designed to pass an SAE reverse double flare into a metric bladder flare master cylinder or equivalent. I needed to run a 3/8-24 line on an old Kaiser/Willis jeep into a weird 3/8 bobble flash remote amp which I added. I modified this by running a 3/8-24 tap over a very similar M10x1.0 male thread as the required adapter was not available. This has worked well so far.