On the reverse, IIVIRAAA can clearly be seen. Shortly after that, I believe the beginning of the legend is C PLOTIVS. This points to RIC I (second edition) Augustus 389, which is a coin minted under the emperor Augustus by the moneyer C. Plotius Rufus in 15 BC. Some of the examples there have countermarks, like on your coin. In your second picture, on the right side, I can just make out the VNIC part of the obverse legend.
I'd still like to know the diameter and weight to confirm that it's an as rather than a dupondius. Numista has pages for both, but the RIC I catalogue, last revised in 1984, lists only the as.
N#247870
N#338451
