Probus became emperor in the aftermath of Tacitus execution in 276. After a number of successful campaigns ranging from the Rhine frontier to Egypt and the defeat of three pretenders to the throne, Probus was assassinated by his own troops near the city of his birth, Sirmium, in Pannonia. The reverse legend and type of this coins are generic in this period, but the purity mark was a new addition of Probus' recent predecessor Aurelian, who had stabilized the purity of the coinage at 5% silver and marked this with XXI as an indication that the coin consisted of silver in the ratio of 1 part to 20. Andrew Carroll 2011.