Hard to describe, I think it's not a metric fitting (no line on the nut section.....bad description sorry) it's for 3/16ths brake pipe into a wilwood master cylinder.
Simon
Simon,
It will either be an M10x1.0, 3/8"-24 UNF or 7/16"-20 UNF whch are standard 200-03 series fittings. I use Speedflow in Snaresbrook, London. They are extremely helpful & prices are competitive.
Try their website.SPEEDFLOW