The civil Assyrian miter (a crown, really, which is what a bishop’s miter is, too) is kind of like a fez, but with tails off the back. Mind you, Assyria fell about 550 BC…
Assyria: her manners and customs, arts and arms: restored from her monuments, p 459 (Google Books result)
It is likely that the original miter of the Assyrian Christians was similar to the civil miters of Persia, which seem to match the description closely - a rigid tall cylinder with crenelations, or a soft pointed cap (called a Tiara).
The later sassanid tiarra was round topped, but similar, based upon the coin images, and a sign of courtly rank.
Note that the Papal Tiara is of similar shape to some of the Sassanid ones…
So, I think the fez-like miter is probably the traditional one. But I can’t be certain. I’m just pointing out the similarity as indicative evidence, not proof positive.
Note that the Sassanid and Persian crowns appear to be essentially the same shape as that miter, but with a large plume (probably ostrich) off the top front.
home.eckerd.edu/~oberhot/sasanian.htm
iranicaonline.org/articles/courts-and-courtiers-ii
iranicaonline.org/articles/crown-i