the century - fighting unit

[SOCL]

There is an interesting snippet from Tacitus which, as far as I am aware, has never aroused scholarly interest. After the second battle at Cremona, the Seventh Legion was found to have lost "six centurions of the First Order" (Hist. 3.22: occisi sex primorum ordinum centuriones); i.e. six of the primi ordines. But there are only supposed to be five!

I was under the impression that there is little certainty about who constitutes the primi ordines and that they may in fact that not account for the centurions of the first cohort. The loss of six certainly means that they are in high demand (i.e. few in number), but I get the impression that there must have been quite a few more than six. A suggestion I've seen in this forum--though I don't remember where and it was quite possibly speculation--was that the primi ordines also accounted for the senior centurions of each cohort, but like I said, this may have been speculation, especially since I don't have documented proof to back the claim. In any event, is there any concrete evidence that the primi ordines constituted only those centurions of the first cohort?