Roman Legion in late Antiquity

[Frank]

Of course it is - because, apart from the Polybius construct we have nothing else of comparable detail. A century of 80 men (10 x contubernia of 8) with a Centurion, Optio and Signifer.

I don't know the original text. Did Polybius really say, that the optio and the signifer have been supernumerarii?
Afaik, the camps just show 10 rooms plus the room for the centurio. This would mean 81 people per centuria.