Well, I got the courage to clean this coin. To my great surprise, I don't believe it was horn silver after all. Lemon juice with a cotton swab cleaned it up nicely. I stopped to examine and ponder whether or not to clean it further. It might be good as is.
Found out some more info on the cin as well. according to
http://www.historicalcoins.com: "This coin was part of a large issue of coinage by two supporters of Pompey who had fled Rome upon Caesar's approach to the city. The SC upon the reverse was an attempt to show legitimacy for their coinage via the senate, but this could only be via those Pompeian senators who had fled Rome, too."
