In his vision, what does the man see printed on the flag?

I'm sorry, this is a short-answer literature forum. We are unable to answer questions about motion pictures unless otherwise noted. I did, however, find a copy of the script which describes a flag in the following way....

As they begin to close in, Algren whirls the lance around, a tattered battle-flag with TIGER INSIGNIA still dangling from the end.

Note, I am not sure if this is the flag in question.

Katsumoto's vision showed him a white tiger on the battle flag, as well as the target of the attacks appearing itself as a white tiger. Shortly after Captain Algren and his soldiers encountered the samurai, Algren is dismounted and wounded. He picks up what appears to be a sashimono attached to a spear, with which he fends off his attackers. Katsumoto watches his defense and notices the same white tiger from his vision, but also notices the ferocity with which Algren fights (perhaps similar to a cornered tiger). As Katsumoto believes Algren to be the tiger from his vision, he stops his men from killing Algren.