I think it may be the legend ARG PUB on Obv. left, which is an abbreviation for argentum publicum or perhaps argento publico, either of which means public silver or public money. It is assumed that the silver for denarii came from the State but this issue is one of only 8 in the entire silver series that specifically says that. The coin is also well struck and has a control mark, which also makes it interesting to me.
Here is something that might take you some time to read entirely : http://andrewmccabe.ancients.info/ A must ! Q
On the obverse are the letters ARG PVB indicating the coin was struck from public silver as a special issue (usually for some special project or emergency) rather than being part of the annual regular coinage overseen by the regular moneyers. I am not full up on the various codes for special issues (EX SC is another). As I recall there are also coins marked AP for the same reason. I don't believe there is a lot in the way of surviving records explaining how the mint worked or accounted for the precious metals used so a lot of what we 'know' is reverse engineered from evidence I do know understand fully. The D under the horses is a variable letter (die?, production date?, whatever???). Banti shows the coin with 16 different letters under. Some are lettered above the horses rather than below.