It could be a mount (phalera) for a sword baldric (shoulder strap).
That's what it looks like, the two-piece
type with the hole for the separate D-ring. However, it is a
bit small. Most documented baldric phalerae are 50-80 mm
diameter, with a few known as small as 40 mm.
Perhaps this is a smaller
provincial version??
It could also be a form of strap mount I guess but round
types with a hole in the centre are not really documented.
SC
