Promotion in a Vexilatio

[Frank]

a lot of centurions were travelling regulary to Rome for promotion and came back with news from the city.

Which period was this? Might the centurions perhaps have been promoted into the Rome cohorts (as many were, after about the mid first century)? They would then return (albeit probably to a different legion) several years later as primus pilus, which would have given the frontier legions a link with affairs in the capital... or was the sense of the comment that the travel back and forth was speedier than that?

Egon Flaig "Den Kaiser herausfordern" (To challenge the emperor) covers the first century AD. Hartmann focusses on the 3rd century.

If I remember right, not so many soldiers or normal centurions were deployed from the legions to the praetorians before Septimus Severus. Primipili could become tribunes of the praetorians. And of course some praetorian evocati became centurions in the legions. Some tribunes of the praetorians became primipilus iterum and were deployed as deputy of a legatus augusti pro praetore.

But that has nothing to do with promoting centurions. Thats not what I have read about.