How should a character address an unmarried woman in conversation in the 1930s: would they use Mrs., Miss, or Ms? Example: "Good evening, Ms. Smith."
In the 1930s, 'Miss' would have been the common form of address for an unmarried woman.
'Ms.' was extant (and had been since the 1600s), but rarely used. Its revival was proposed in a 1901 newspaper article as a neutral replacement for terms denoting marital status:
The abbreviation "Ms" is simple, it is easy to write, and the person concerned can translate it properly according to circumstances.
(See 'Hunting the Elusive First "Ms."'.)
The proposed revival, however, did not gather steam until the early 1970s, and the absence of involvement of marital status in the term 'Ms.' remains a point of confusion for many to this day.
'Ms.' has been in use, along with 'Mrs.', 'Miss' and 'Mr.' since at least the 1600s, as (less formal) variants of the formal titles 'Mistress' and 'Master', which last two did not originally convey any information about the marital status of those so addressed. About 'Ms.', one historical researcher says
It’s curious that the use of Ms is often criticised today as not ‘standing for’ anything. In fact, it has an impeccable historical pedigree since it was one of several abbreviations for Mistress in the 17th and 18th centuries, and effectively represents a return to the state which prevailed for some 300 years with the use of Mrs for adult women – only now it applies to everyone and not just the social elite.
(See "Mistress, Miss, Mrs or Ms: untangling the shifting history of women’s titles" in the 12 September 2014 edition of the New Statesman.)
Definitely "Miss" if followed by her surname.
I would suggest "young lady" when addressing an unknown yet well-dressed girl.
"Mademoiselle" still has its charm, and did back then as well.
In certain ultra-progressive circles young women insisted on being addressed by the surname only, on both sides of the Atlantic.