OK, now I know its legal you can send me the reference in slow time, as per your PM.
I knew this was going to be a tough one, & one of my leading Roman experts (advised time-team), book writer etc... has pretty much drawn a blank;
''Thanks for the note and the images . Always difficult to deal with an object with just a couple of images but it looks more Roman than medieval to me with cast or even chip carved decoration? There is a curious 'slot' at the back of the object which looks to have housed another part of the object- so its not all there. How about a lynch pin? The slot could have taken an iron pin whilst the AE part that you have would formed the part that was at the top of the pin and formed the decorative top plate. The object is too big to be some form of purse mounting.
Let me know if you have any further ideas.''
The lynch pin idea is not too far of my thoughts that its some kind of chariot/wagon fitting, so I've thrown out another couple of emails & lets hope we can piece it together. But like I stated, I believe this to be very rare & most likely undocumented, so we may never get the truth of it. Other than I'm 100% sure its Roman period. Both bull's & swan heads are common decorations.