The item is very unlikely to be a sword pommel. Swords were worn on the back in fantasy novels, but not in the
Roman period when they were worn at the hip on a waist belt or at the
side between hip and shoulder slung on a shoulder baldric, which always
had a strap that passed diagonal over the chest.
Every other
fibula that I have seen on a
bust is set lower, on the shoulder
part. Plus this is comparatively huge. And it looks like no
fibula.
Though it looks odd I suspect it is bow-related. Top of a quiver - though shape is weird. Fletched end of a bundle of arrows in a quiver? Perhaps the end of a bow case?
SC