A small and useless footnote for Danny and NigG, from a longtime devotee of this and the Brevet threads: the term Missy-Sahib was in use in Rangoon as late as 1978-79.
At least, it was still in use by my cookbearer, Samson, who had come from India to Burma in 1945 with a British Army Major, and who had later worked for a succession of British Defence Attaches until the DA post was chopped in 1978. (Which made it suddenly hard for a now civilian-staffed embassy to maintain channels to the military regime. But Whitehall knew that of course.)
Samson, a slight, wiry but intensely loyal fellow from somewhere around Darjeeling, used the term whenever he spoke to any unmarried female European visitor to my house under the age of about 25. And occasionally a married one if he was in doubt. Anglo Indian or Anglo Burmese young women were not so honoured!