Here are examples from each officina. Let's see your coins of Gallienus or Salonina with officina marks! At first, the Rome mint used a letter abbreviation for the Latin number of the officina, such as P, S, T, or Q (prima, secunda, tertia, quarta) for the first four officinae, and the Roman numerals V and VI for the fifth and sixth. They could not use the letter abbreviation for "fifth," quinta, because it would have been indistinguishable from Q for quarta, or for "sixth," sexta, because it would have been indistinguishable from S for secunda. P (=Prima, meaning "first"): S (=Secunda, meaning "second"): T (=Tertia, meaning "third"): Q (=Quarta, meaning "fourth"): V (="fifth"): VI (="sixth): But by late in the reign, the number of officinae was expanded to 12, necessitating a change in the numbering system. As Jim Phelps* explains, "officinae numbers 1-8 used Greek numerals, while 9 used Nu (N), which normally meant 50. The normal Greek letter for 9 was Theta (Θ), but this was also the first letter of the Greek word for death, Thanatos, and seems to have been considered unlucky. Officinae 10-12 went back to typical Roman numerals, providing a mixed and sometimes confusing pattern." I personally think Phelps errs in his explanation for the ninth officina; the most likely explanation for using N for nine is not that it is the Greek letter nu, but that it is an abbreviation for the Latin nona, meaning "ninth." Phelps, discussing coins of the so-called "zoo series," explains that "each officina produced a different coin within the series, with some producing a second, less common type also." He provides this table, which makes this readily apparent: A (Alpha, officina 1): B (Beta, officina 2): Γ (Gamma, officina 3): ~~~ * Jim Phelps, “NumisWiki - The Collaborative Numismatics Project - Thousands Of Online Numismatic Books, Articles And Pages. Gallienus+Zoo.” Forum Ancient Coins, www.forumancientcoins.com/numiswiki/view.asp?key=Gallienus%2BZoo.