Third Century Legionary Legates

[Renatus]

There are also a few inscriptions to the military staff of a legatus legionis ii parthica at Apamea, so clearly at some point the legion briefly gained a senatorial commander.

I imagine that you are referring to Aurelius Tato, strator leg(ati) Leg(ionis) II Pa( r)thicae, Verinius Marinus, librari off(ici) leg(ati) Leg(ionis) II Parth(icae) P.F.F. A(e)t(ernae), and Aurelius Hermodorus, exactus librarii legati Leg(ionis) II Part(hicae) Anton(inianae) P.F.F.A.. There could be another explanation for the reference to the legate in these inscriptions. The full title of the prefect in charge of this legion was praefectus vice legati (CIL 08, 20996), implying that he was deputising for the legate. Whether or not this was truly the position, the notion that the post was temporary and that the legion could expect  to be commanded by a legate in the future would mean that it was quite in order for the headquarters staff to be called the officium legati, as in other legions, and for the commanding officer's groom to be the strator legati, even though, for the time being, the officer employing his services was a prefect.