That SC on the reverse is a considerably thicker font than I've ever seen for this issue and there are hundreds of examples of it at acsearch.info.
My sense is that this coin has a couple features that look less-than-authentic to me: 1. As mentioned, the S C on the reverse looks too broad and flat, especially when compared to the VESTA on the reverse which looks normally struck with accompanying wear from circulation. 2. The obverse portrait of Caligula looks a little off -- especially the chin which appears to be more prominent than most of these portraits. It's possible that the S C was accidentally filed flat during the harsh cleaning process, since some of the obverse letters appear a little flat too. Personally, I'd look for a better example from a VCoins dealer.
I won this auction for 20$ USD , i do not really have the budget for a better Caligula but if this is clearly a forgery i will notify the seller i wont pay.
I think it's a copy of this, just yours is made to look worn/trashed. http://www.forumancientcoins.com/fakes/displayimage.php?pos=-3602
I don't know about the authenticity, but I certainly do not like the looks of it. Something is off besides just the lettering.