Cooks of any seniority were always 'Mrs'. Housemaids and parlourmaids were supposed to have 'suitable' names - e.g. Jane, Mary, Edith, etc. Such names as Violet, Muriel, Rosamund and so on were not considered suitable, and the girl was told firmly, 'Whilst you are in my service you will be called "Mary"'.
Source: wiktionary