Diocletian: legions and rank structure

[Nathan Ross]

He then cites CIL V 1721 as a proof.

I can't find this one translated anywhere - does it mention centurions, necessarily?

CIL 05, 01721 (Aquileia): Debita non optata dies iuvenili advenit aetati / cum meritis gauderet suis qu(a)e olim labore quaesiit / tunc desertus dulce est coniugali vinculo / ac post non magno et crudeli tempore / eorum fata durae(!) coniunxerunt corpora / uno iacent ambo non toro sed tumulo / nec remisere saltem subolem qui meminerit suos / quod non quidem saolis nec nunc primum contigit / omnibus semper suis fuit carus amicus / hic ducenae dignitate inter lectos meruerat viros / huic contubernales sui id contra votum posuerun(t) / fors qui intendit doleat sed sic se fata ferebant

All the officers that Hepworth lists as ducenarii appear to be from the auxilia or cavalry.

I would love if someone did a case study of Leg II Italica Divitensium

Yes! Ross Cowan gave a brief resume in his latest Osprey book (and may do a less brief one in his forthcoming title on Milvian Bridge). The idea that their Italian tombstones follow the course of Constantine's 312 advance is too tempting to ignore really - I managed to find Baudio's stone on a recent visit to the museum in Spoleto.

Besides their interesting name evolutions - and the fact that one of them was a recently-recruited Raetian - they seem to have had an unusual habit of stating their cohort number on funerary inscriptions rather than their centuria... which raises all sorts of interesting possibilities about the shape and size of the later 'field army legion', perhaps!