This issue represents the fourth and last issue of the year beginning Dec 10, 165 - December 9, 166. What distinguishes this issue from those preceding it is that Marcus has received his fourth imperial acclamation, and all of the reverse inscriptions of this issue read TR P XX IMP IIII COS III. On the coins of this issue, as well as the preceding third issue of 165/66, the obverse inscription includes the title Parthicus ("Conqueror of the Parthians"), a title which first appears on coins issued from August - December 9, 165 (the previous year). These changes in titulature reflect the events of Marcus Aurelius's and Lucius Verus's five-year war with the Parthians. In AD 166, there was one final campaign, which established Roman superiority. Avidius Cassius, commander of III Gallica (one of the Syrian legions), crossed the Tigris and entered Media, sacking the twin cities of Seleucia and Ctesiphon, which was the capital of Parthia. A peace treaty was signed and Marcus received his fourth imperial acclamation (IMP IIII). Both he and Lucius Verus also received the title of Medicus ("conqueror of the Medes, not "medical doctor"). You may read all about the war here. All of the coins of this last issue of of the year ending December 9, 166 have either Pax (Peace) or Victory on the reverse, and serve to publicize the military accomplishments in the war on the eastern frontier. It's significant that Victory's shield is inscribed VIC PAR, short for "victory over the Parthians." Your coin was struck some time in autumn, AD 166. It was probably issued around the time of the triumph in October to celebrate the victory. This triumph was celebrated jointly by Marcus Aurelius and Lucius Verus on October 12, 166. Both emperors acquired the title Pater Patriae, and, at Lucius' request, Marcus' sons, the surviving twin Commodus and his younger brother, Annius Verus, were made Caesars. And as if Commodus being declared Caesar wasn't bad enough for the Roman people, the praetorian guard brought some sort of plague from the east with them to the city.
wow!..i'd read about the plague of that time, but i had no inkling about the coin, i am more enlightened now , thanks RC! you are a gentleman & scholar