Late Roman Unit Sizes

[Nathan Ross]

the theory the iuniores were originally the prior centuries and the seniores were the posterior centuries.

You might be interested in this quote, from de rebus bellicis (5,5):

scilicet ut centeni aut quinquageni iuniores, extra hos qui in matriculis continentur, habeantur in promptu armis exerciti et minori utpote tirones stipendio sublevati, in locum amissorum si res ita tulerit subrogandi

I can't find a translation and don't know latin, but Richard Duncan-Jones (Structure and Scale in the Roman Economy) claims it "suggests that gaps in the standing army should be filled" by the recruitment of subunits of 50 and (?) 100 men. This, Duncan-Jones says, "suggests that the writer was thinking of unit-sizes current in his own day". I'm not sure whether the mention of iuniores is at all significant...