Leather Armor? (NO HOLDS BARRED!!)

[Theodosius the Great]

Assumptions like that are dangerous- I wouldn't make it myself.

In this case, I disagree. Everyone else is wearing armor (musculata). The picture is of the famous Louvre relief.

Is there any unrelated reason to think that anyone ever wore a kind of surcoat over his armor?

AFAIK, this is the only occurance. Please prove me wrong though, anyone .

From the way the tunica is hanging, it looks to me to be simply body form underneath- not armor which would be clearly larger than the man wearing it

He could be wearing a musculata. How else do you explain the pteruges ? You could assume he's merely wearing a subarmalis underneath, but as you said, assumptions are dangerous.

Are those men actually soldiers or are they praetorians or ceremonial guards?

Good question. They are called Praetorians for no apparent reason. The most one can say for sure is that they are "officers".

I believe the issue is with some kind of extra layer of protection- specifically leather, not just any old thing on top or armor, right?

Well, Dan said "anything" (other than a cloak), so....

It's an interesting picture though

Yes, it is in that it's unique, but not all that helpful, perhaps.