Well if yours is right Alan, then mine is wrong!
I'm not 100% sure mine is right but I found this on Keith Calver's website and went with it;
The later master cylinder (GMC227 - supplied as a replacement for the early types and requires a conversion pipe kit, sometimes identifiable by a yellow plastic band around the cylinder body) has metric threaded fittings with nuts of two different sizes - top one bigger than the bottom one. The conversion pipe kit is necessary because the piston functions (and therefore the port supply ones too) are reversed. The upper/top port feeds the front brakes, the lower/bottom one feeding the rear brakes.
Made sense to me because the smaller bore in the top part of the cylinder will produce more pressure so should probably feed the fronts.
Hope that helps!