The modern usage of "Ms." as an English-language honorific for a woman of indeterminate marital status was coined in 1961, and not popularized until a decade later, when feminist Gloria Steinem used it as the title for her new magazine.
While there were some interesting antecedents ([[http://en.wikipedia.org/wiki/Ms
. described here]] by The Other Wiki
, the modern usage of "Ms." in fiction is generally anachronistic and inappropriate for stories set prior to the 1970s.
A lot of modern writers either forget this or are ignorant of the relatively recent origin of the honorific.