My wife’s grandmother was born Grace Alice Mortier in Rock Island County, Illinois, in 1913 and married Wilbur Johnson. In many records after her marriage, she is referred to as “Grace M. Johnson,” with the “M” standing for her maiden name. Unfortunately some have seen the reference to her as “Grace M.” and assumed that her maiden name was Grace M. Mortier–with the “M” standing for another name.
Some women used their maiden name as a middle name after their marriage. If a married woman’s middle initial is the same as the initial letter as her maiden name, that could be what the letter is standing for.