Mashka is the most plausible if the girl is relative low-born. 17th century etiquette allowed this."Mashenka" is just not a formal address. The formal address will be "Maria". "Masha" is a short version. "Mashka" is a variant of a fraternal appeal to a friend or a teaser. "Mashenka" is an affectionate address. And all these are derivatives of the name "Maria".