With an Antoninianus of Probus with Salus reverse I have completed a set of 6 coins with a hidden message. I first learned of this series from Doug Smith's page, EQVITI Coins of Probus. It was a while ago and I think I used CompuServe to connect to Geocities at the time.
Background
Let's start with this coin of Postumus, issued by Auréolus in Milan. Auréolus who from circa AD 258, during the reign of Gallienus, had been Magister Equitum, or chief general in charge a new cavalry unit at Mediolanum (Milan). He turned on Gallienus and joined forces with Postumus minting this coins in Mediolanum in 268. As a cavalryman, perhaps not surprising that he celebrates the Cavalry. "Virtvs Eqvit" celebrating virtue, bravery and military strength of the cavalry.
Aureolus, Romano-Gallic Usurper, AD 267-268, Antoninianus, Mediolanum (Milan) mint, 3rd officina, struck in the name of Postumus, 3rd emission, mid AD 268
Obv: IMP POSTVMVS AVG, radiate, draped, and cuirassed bust of Postumus right
Rev: VIRTVS EQVIT, Virtus advancing right, holding transverse spear and shield; [T] in exergue
Fast forward 7 years: Claudius II, became the new Magister Equitum for Gallienus, took out Aureolus, then took out Gallienus and became emperor. Claudius II died of the plague, and his brother, Quintillus briefly became emperor before losing support in favor of Aurelian. Aurelian is betrayed by his officers in 275 on his way to take on the Sasanians, and in quick succession Tacitus, and then Florianus are dispatched, leaving Probus emperor of the restored Roman empire.
The Coded Series
Probus rose to emperor from the equestrian ranks. The Equites which are often translated as knights, were a class, and inherited rank in Rome below the senatorial class. The authors of RIC (Roman Imperial Coins) note that "it is not unlikely that Probus, whose warlike operations extended over a great area, may also have owed much to the mounted arm, and desired to express that indebtedness. That he himself was no mean commander of the horse may be indicated by the title Equitius Probus, applied to him in the Epitome". The latter a reference to this line:
"After him, Tacitus took power, a man of singular character, who died at Tarsus from a fever in the two hundredth day of his reign. Florian succeeded him. But when the majority of the troops chose Equitius Probus, a man experienced in military affairs, Florian, on the sixtieth day of his reign, as if exhausted in the contest for power, when he had cut open his veins, was consumed by loss of blood."
A series of coins for Probus have a hidden message that was first published in 1873: Missong, A (1873) "Gleichartig systemisirte Münzreihen unter Kaiser Probus", Numismatische Zeitschrift (1873), Wien, p.102-115
Missong' s interpretation has been superseded, however, he read this code as coinciding with improved mint consistency and quality standards. (Tarraco has since been replaced with Ticinum as the mint for these coins.)
"Both in Tarraco and in Rome there was a lot of variability in the manufacture of the mint dies. During the third consulate in Tarraco, greater regularity in obverse and reverse appears to have been introduced, and at the same time, as a sign of equality (AEQVITAS) in the six offices, each of them took one of the first six letters of AEQVITATI as a symbol."
He remarks that in contrast with the previous emissions, in these coded coins there is remarkable unity and equality of the reverse, without variation. In the obverse, the bust of the emperor always facing right with curaissed and radiate (4th emission), and always facing left with imperial cloak and the radiate crown, or curaissed with a radiate helmet (5th emission).
He rejects "AEQVITI, as a designating money for the cavalry or knights" - its absurd that Probus would only issue money for the knights. He also dismisses, the idea that the code might be a name for a mint manager, given the same code is used in Rome and Ticinum.
Here is my set from a mix of emissions (Bastien numbers as 8th, 9th with and 10th without star) and a mix of officinae (1-6) - ordered by officina (P-primus, S-Secundus, T-Tertius, Q-Quartus, V - Quintus, VI - Sextus) and with the coded letters spelling "EQVITI". For more on the reverses & workshops, I will refer you to Chip Scoppa on Forvm. There are three series from Ticinius and another one from Rome. The thrid series includes a star. The 8th series spells AEQVIT.
Momsen, his contemporary, had a different view on the meaning of the code:
"It seems evident to me that this is the name of the official who directed the coinage under Probus - Equitius or Aequitius, both spellings are found, but the first predominates - it is not an uncommon cognomen in the third and fourth centuries, and this Equitius may be an ancestor of the consul of the same name in AD 374."
-Momsen, T. (1887), ‘Equitius.’, Zeitschrift für Numismatik, 15, p.251-252
The rationales for the code described by both Missong and Momsen have both been overridden by other hypotheses. Note that I didn't say we have any definitive answer - there may have been no more meaning than "useful set of letters for mint control". There might be another hint in some rare Antoniniani referenced below with a horse head on the obverse. For a thorough discussion of the various thoughts and evidence - see this excellent page on forumancientcoins from Gert Boersema, and probvs.net for a listing of all the relevant coins.
Additional References
Here are a couple of other relevant papers on the topic of Cavalry and Probus in addition to the ones linked in context above:
VIRTUS PROBI: PAYMENTS FOR THE BATTLE CAVALRY DURING THE RULE OF PROBUS (A.D. 277–278)*Fernando López Sánchez
Margetić, D., & Margetić, D. (2012). Rare and unpublished Siscian Antoniniani of Emperor Probus with Horse Head on the Obverse.
The different series are well explained by Chip Scoppa