\"Celtic\" military technology and the Romans

[Dan Howard]

Further, the Dura graffito has just the anomaly that you complain of; the arms and legs are protected by articulated armour but the breast is protected by scale or, possibly, mail.

We don't actually know this. There was Persian padded clothing that looks very much like the Dura Graffitto and other contemporary illustrations such as at Firuzabad. There are two main interpretations of the Dura Graffitto - either the limbs are covered with segmented plate or they are covered in padded cloth.

If you take Ammianus' statement as factual, "And the light circular plates of iron which surrounded their bodies, and covered all their limbs," then some sort of anime cuirass is being described, also not merely mail or scale.

Scale armour can be made from small circular plates. There is alo "bezainted" armour where circular discs are attached to a backing, like scale, but do not overlap, unlike scale.