In addition to TIF's fine explanation: Not all rulers used the same dating 'rules'. Caracalla was son of Septimius Severus and used the dates of his father even after his father died. That means the first year of his sole reign shows as year 20 on his coins. Commodus did the same with his father so his last coins were year 33 even though his reign was under 18 years. Faustina II has coins for years 11 through 24 using the dates of her father (Pius) and later coins 4 through 10 using dates of her husband (Aurelius). That is right, 24 is earlier than 4. Compare her apparent age on this year 12 https://www.cngcoins.com/Coin.aspx?CoinID=333533 and this year 5 https://www.cngcoins.com/Coin.aspx?CoinID=329916 Severus Alexander has coins as Caesar showing year 5 of Elagabalus but started over using his own numbers and has another year 5 later. Maximianus became co-emperor one year after Diocletian but started with year one in that same year Diocletian was using year 2. This continued through their reign so coins of Diocletian for any year show one number higher than the Maximianus for that same year. Worse, when they added Galerius and Constantius Caesars they started with year 1 so the last coins made in April 296 are year 12, year 11, or year 4 depending on whose face is on them. The first coins of Septimius Severus are year two because Pescennius Niger controlled Alexandria until after August 193. There are Year 1 and 2 Pescennius coins (I don't have one but compare the prices of these two CNG sales of the same coin) https://www.cngcoins.com/Coin.aspx?CoinID=70386 https://www.cngcoins.com/Coin.aspx?CoinID=349941 When Pescennius was tossed out, Septimius started with year 2 making their coins match. Since this changeover happened in February 194 we expect Pescennius LB coins to be September 193 to February 194 and Septimius LB like the one below to be February to August 194. What I'm saying here is that, if you want to collect coins of Alexandria, consider buying the book (but you may find it cheaper than from Amazon): https://www.amazon.com/Alexandrian-...=1521837678&sr=8-3&keywords=emmett+alexandria
Thanks @Bing. The output of bronze coinage from Trier was quite plentiful, so it’s possible to get a nice example at a reasonable price.