Speculatores

[M. Caecilius]

Very odd! I've never heard of naval troops being organised into cohorts, unless they were operating as land-based auxiliary units. I suppose there may have been speculatores attached to the fleet - but could the inscription be a bit garbled? Centurion of a praetorian cohort, attached as a speculator to the fleet, perhaps?

Considering the inscription does not appear in the Epigraphy Databank, which includes most of CIL and AE, I would assume it has been lost, which seems to have been the fate of quite a few inscriptions before (and since) CIL was first published, a century after Eckhel. Simple google searches for M. Staberius also offer no results but references to Eckhel. However, it's interesting to note (as Eckhel does), that for both this inscription and the coins, the context is naval, and refers to cohorts. Eckhel does mention that speculatores were attached to the fleets as swift scouting ships and lookouts, rather than undercover or plain-clothes spies.

But I don't see evidence for full cohorts of them. The Antonian coins relate to a very specific late republican organisation, with the praetorian cohorts raised from veterans (originally all centurions, says Appian), so the 'cohortis speculatorum' could just an oddity of the era.

Probably, but it is weird that they were important enough to be reckognised by Antony as a unit apparently separate from the legions, and due some special status. The accounts of Actium itself also do not seem to say anything about this. It'd be interesting to know why Antony, if it was his initiative, sought it necessary to organise a specialised corps of army or naval scouts, and if it was not his initiative, who used them. I do not think Caesar ever speaks of them, as far as I know, for the Gallic or Civil Wars, nor does Augustus seem to perpetuate these cohorts in his military settlement.

Not sure - these could be centurions of the speculatores augusti (also mentioned in inscriptions), which I think were part of the praetorian guard. Alternatively, the speculatores of a legion could be commanded by a centurion, maybe.

As most inscriptions I could find in this context are from Rome or Region I (Latium et Campania), my guess would also be that they are members of the praetorian cohorts. They did seem to have centuries, so they would be somewhat separate units, if the inscriptions are anything to go by; whether they would fill out a full cohort cannot be said with the available evidence being limited to a coin type from 31 B.C. and a now-lost inscription.