The old-established Praetorian Guard was based at the Castra Praetoria in Rome, and had frequently proved disloyal, making and deposing emperors and even on one occasion in 193 putting the Imperial throne up for auction to the highest bidder (cf: Didius Julianus).
The majority were struck in the reigns of emperors Constantius II and Julian and derive from a range of mints including Arles and Lyons in France, Trier in Germany and Rome.