Look at the helmets on the bottom picture from these 5th century mosaics http://diaconia-idc.org/blog/projekte/the-tri...risti.html Very similar to 9th century art. The 4th mosaic down might be showing more detail. Here it looks like the brim comes down. Usually there's not enough detail to show how the front is designed. It also looks like they're four piece construction. Once again, these are bible scenes and we don't know if they're contemporary helmets.

The similarity to 9th century art isn't co-incidental. Late Roman art was the common model for Carolingian art. This makes it difficult to be sure what was actually worn in the 9th century. Did they dress in a Roman style to deliberately give the impression of "romanitas"? Or did they just appear that way in art?

Things are made more complex by the fact that Roman art could be quite conventional and may not have accurately depicted, say, helmets. You could read all the helmets in those mosaics as stylised crested ridge helmets. The bottom most ones even have the eye markings commonly found on this type. Whether the two red-crested helmets with the prominent brow reinforcement represent an actual sub-type we don't currently have an archaeological example of or are an artistic reference to an earlier type of helmet remains, I think, a point of discussion.

Author Timothy G. Dawson believes this is showing a Byzantine quilting pattern: https://manuscriptminiatures.com/4868/13186#image He could be right. I noticed the lines on the upper arm. Compare it to whatever this guy’s wearing under his scale: https://manuscriptminiatures.com/4868/13175#image I know there’s some pretty abstract attempts at portraying mail this early, but that second image looks like he’s wearing something under that armour. One problem I have with the Stuttgart Psalter is the wide pants: https://manuscriptminiatures.com/4868/13164#image You see those in Byzantine art: http://warfare.tk/6-10/Maximian_Throne-Joseph_and_Jacob.htm