Five different types of classic Mini master cylinders were used from 1979 to May 1985, and can be identified by a rectangular reservoir, pressure failure warning switch and .70" diameters of both primary and secondary bores. All cylinders are now obsolete, but all use the same repair kit if the body is rebuildable.
If your cylinder is not rebuildable, it can be replaced by GMC0227 with appropriate pipe conversion kit.