Late Roman Unit Sizes

[Nathan Ross]

I need to find if there are inscriptions past Diocletian’s reign mentioning prior and posterior centuries so as to put this line of investigation to bed once and for all.

As far as I know, there are no inscriptions mentioning centuries at all post Diocletian. I think the last dateable ones are related to legionaries transferred to the praetorian guard, perhaps by Maxentius. The decline of epigraphy generally during this period leaves little evidence to go on, but it seems possible that the century was phased out under Constantine. Vegetius only mentions it as part of his 'antiqua legio'.

Roger Tomlin... thinks that the (VIII Augusta, i.e. Octavani, based in Italy) was the 7th unit sent which I think was a legion, not the Augustei

Could be true, although why should a poet be more aware of the 'Augustan' unit title than the compiler of the official Notitia? I think it's more likely to be the auxilia unit, just because it's listed among others of the same type - Claudian calling it a 'legion' creates confusion, but he's previously referred to the Herculiani and Joviani as 'cohorts', so we shouldn't put too much faith in his terminology!