http://en.wikipedia.org/wiki/Military_uniform
The regular thematic (provincial) and Tagmata (central) troops of the
Byzantine Empire (East Roman) are the first known soldiers to have had what
would now be considered regimental or unit identification.
不曉得為什麼這裡會用現在完成式

強調"到目前為止"被認為是....？(不確定)

Because the troops have had that identification for awhile如果改用過去式最好要加上時間抱歉,解釋上應該是had had,因為這軍隊應該已不存在