I've made it to the reign of Marcus Aurelius or so and I've come to a unit and an inscription that I'd forgotten about. CIL 3.14217, 6 (AE 1901, 21; IMS 1.119). It lists an Aurelius Victor, a soldier of the c(ohors) II aur(elia) n(ova) sacor(um). There are a few interesting things about the unit mentioned but I want to highlight two: first (A), this is the only record we have, so far as I can tell, of this unit; second (B), the identity of the mysterious "Sacor(um)".
A. There is at least one other cohors ii Aurelia based in Moesia Superior, maybe three others. As noted, there is no other record of the "sacor" variant. That in itself is noteworthy. Why only here? Is it a mistake? On the other hand, the "sacor" seems distinct enough that it seems unlikely to me that it's a lapicidal error. Did the unit later get this title for some reason that we can no longer deduce?
B. Most readers of the inscription are happy with "sacorum" as the unabbreviated form, the genitive plural, well, of what exactly? Some have suggested that the people it refers to are the Sacae, though the genitive plural would not then be Sacorum, but Sacarum. So, lapicidal mistake after all?
If it is a people, however, and there's plenty of precedent for including the genitive plural of a people in the nomenclature of a unit, who are they? I've found vague references to Sakai in the second century (Ptolemy and Aelius Aristides). Some modern works use this name/term too. But we also find references to the Saka or Sakas people in modern studies. Are these the same people?
Those who stick with Sakai tend to classify them as eastern Scythians. Those who refer to them as Saka/Sakai tend to associate them with the Sarmatians and give them an inner Asian background. The two-Scythian and Sarmatian-aren't necessarily mutually exclusive, and given the confusion often found in Graeco-Roman sources with respect to barbarians such a mistake is not surprising. Still, more information would be warmly welcomed.
In the end, my preferred theory is that these "sacorum" are part of the contingent of Sarmatians that were compelled to fight for Rome after their defeat in the Marcomannic wars. I only wish that I had more to base this rather flimsy conclusion on. Any suggestions out there?