Sear isn't much help with this. From what I could find online, this countermark is associated with Tiberius Claudius Augustus (41-54 A.D.). The coin is about 20 mm in diameter and has a bluish-gray patina. Since the coin I posted a little while ago seems to be of questionable authenticity, I'm wondering about this one.
No, these date to the reign of Tiberius. But, the "Museum of Roman Countermarks" calls this a modern fake. The stamp, not the coin. http://www.romancoins.info/CMK-curiosities.html They don't offer nearly enough proof, though. So until I see enough evidence to sway me to the contrary, I would call it genuine. For more accurate dating, check BMC I (British Museum Catalog of Roman Imperial Coins).
