Part numbers would suggest that there are a number of different master cylinders. There are ones with imperial pipe connections, metric connections, pressure warning switch and ones without pressure switch. The bore can be around 17 mm , 19mm or 20mm. Most are 19 or 20mm bore and metric pipe fittings.
Editor of the VVOC Magazine ReVival
"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.
All disc brake HC models used the same cylinder regardless of engine size with or without pressure failure switch, front drum brake models used a different bore size cylinder again with or without pressure failure switch