Speculatores

[Nathan Ross]

The evidence seems to suggest that there was some kind of group feeling, beyond merely special duties assigned to legionaries...The schola also seems semi-official... more of a 'club' with a religious dimension.

Definitely the sense of a select or elite group - as they would be, if there were only ten of them in a legion, and presumably appointed on merit. The idea that Pertinax was promoted from the speculatores to the frumentarii is interesting too.

it might be that he was centurion of the 6th Cohort (Praetorians, as the Urban Cohorts were numbered consecutively to the Praterians) and at a different time, speculator of the Misenum Fleet.

Yes, that does seem a more reasonable interpretation. I've come across other inscriptions to the Rome cohorts that don't actually state anything more than the cohort number.

So could the full text run as follows?

"M. Staberius,(centurio) COH(ortis) VI SPECULAT(or)CLAS(sis) MISEN(ensis)"