Russian (old spelling) is 18-19th century variant of Russian language. It has consistent alphabet until 20th century. So, you can find information about it in any open sources such as Wikipedia. It is exact period of time.
FineReader Engine internal name: RussianOldSpelling
Russian (with accents marking stress position) is used for recognizing the words which include accent, such as "замо́к" or "за́мок".
FineReader Engine internal name: RussianWithAccent
You can set all three Russian language types available in FineReader Engine as a recognition parameter. But to increase the OCR quality we recommend to set only the language(s) which is(are) present in the source documents.