AE 1981 777

[Ross Cowan]

Wouldn't he need to be a centenarius procurator, in that case?

Not in this late period. As you probably know, Brunt proposed that prot. ducenarii = primi ordines:

http://www.jstor.org/stable/40310480 - go to p. 70.

I'm not sure I agree that ducenarius = primi ord/senior centurion, but I find it persuasive that ducenarius and related terms like centenarius had lost their original meaning and were granted as honorifics to certain ranks. I'll probably have changed my mind by the time I finish Roman Legionary AD 284-337. ;-)