Neither of the above strikes me as correctly addressing the situation. The art
style of the Empire changed over time and is reflected in coins as well as other media. Increasingly multiple mints made some
portraits of different rulers from one
mint look more like each other than
portraits of the same ruler from several different mints do. As I become more familiar with the coins of any period I become more comfortable recognizing individuals by name and
mint. This would seem more necessary as the general art
style leaves photo=realistic whatever the time period. I'm not sure the various mints of
Galba in the first century really look more like each other than do the various mints of, say,
Constantius II. Certainly short reigns might make die cutters less likely develop a
good feel for a
face before it is replaced by another ruler but aging also
comes to play on long reigns requiring following the appearance development over periods up to forty years.