The Roman Army First Cohort and its possible origins

[Imperial Dave]

https://www.academia.edu/30802577/The_Roman_Army_in_Detail_The_Problem_of_the_First_Cohort?auto=download

this is lifted from the academia website but is originally an article in the 'Ancient Warfare' magazine publication

[Patrick Waterson]

Josephus (Jewish War III.5.5) solves this one: the 'missing' sixth double-century is the legion commander's bodyguard, "Those foot-men also that are chosen out from the rest to be about the general himself".