Late Roman belt

[Robert Vermaat]

Also, when not wearing armour, the belt does not need any cross-straps to support it, this only occurs when it's worn over armour.

Is there any evidence that these belts were worn over armour?

Well, from the same Piazza Armerina...
[attachment=3575]2.jpg[/attachment]
Apart from that, with all the bits and bolts attached to such belts, it would be fairly impractical to wear your armour over it.

This is also why I think that the Piazza Armerina mosaics show baldrics, not belt supports.

If they are baldrics, where are the scabbards?

Good question. None are to be seen, even when we are certain that the figures depicted are soldiers. Most of the time there's something in front (a man, cloak or shield), but when not, there's nothing to be seen. I think it has more to do with the artist not showing scabbards (or swords). A few spears are shown, more shields, a helmet or three, but no swords/scabbards.