AR23 - Antoninus Pius L ΙΘ

Year 19 of Antoninus Pius's reign, corresponding to 155–156 AD, falls squarely in the middle of Alexandria's most prolific billon tetradrachm output. The Alexandrian mint operated under Roman supervision but retained its own dating system tied to the emperor's regnal years — a practice unique among Roman provincial mints that continued until Diocletian's monetary reforms effectively ended the autonomous Egyptian coinage in 296 AD.

Billon quality across Alexandrian tetradrachms degraded steadily through the second century, though Antonine-era pieces generally hold better silver content than those of his successors. The L ΙΘ date formula — L being the Egyptian symbol for "year" — is the primary attributive marker distinguishing this regnal year from adjacent issues.