possibly modernize O. S. (old-style) abbreviation

[jaredh159]

There are a lot of O. S. designations of dates in these volumes. This is probably not at all clear to most modern readers. Perhaps in the modernized edition at least, we could expand this to (old-style) or something like that, or add a footnote, or both -- or maybe just convert the dates and remove the designation?

Whatever we decide, it might be good to apply this convention to the rest of our modernized books as well.