Here is the problem when someone writes a book and guesses. I prefer the Socia option but that is just my guess. Until we find further evidence, we know that there was SR on the coins and that is about all. My gut feeling has never been comfortable with the Senate reading since it seemed strange for a city to use that in a time of a strong emperor. Such feelings are, in the end, just guesses, too. All this is fine as long as we realize that there is a difference between knowing and believing.