Since the beginning of the Roman Empire the bronze as was the most common coin in circulation. It was not only minted in Rome but also in the provinces. The as always bore the bareheaded portrait of the emperor on its obverse, and on the reverse the letters S.C for 'Senatus Consulto,' 'by decree of the Senate'.
Over time many of the province mints were shut down. The emperors focused on striking silver and gold coins; the common people's need for small money did not bother them. Hence the amount of change decreased quickly, and before long the provinces suffered from a shortage of minor coins.
As a consequence under the rule of Emperor Claudius (41-54 AD), masses of bronze coins were counterfeit. Additionally genuine bronze coins remained in circulation as long as possible: the asses of Emperor Augustus circulated decades after his death. Many of these coins were later countermarked in order to revalidate them for circulation under the rule of another emperor. The as shown here was countermarked in the Roman military camp of Vindonissa; the stamp bears the letters BON for 'bonum' (good).