Gladius mainzensis or Pompeii Scabbard locket bar fragment. The bar was used to hold a ring in which the shoulder strap was attached to, usually above and below the locket, such as this one.
These were commonly used from a long period during the Roman era until the scabbards switched to a no locket style with simpler scabbard slides of the later period.
There are also a wide way in which these bars were made, from punched to cast. This example is cast and has then been shaped. Clear hammer or chisel marks are visible on the reverse side of the bar where the curve and ridge was created.
This particular fragment clearly has been broken at the weak spot of where the hole was put to attach it to the scabbard and where the metal was bent to allow the ring.(1,2)
The following sketch from Mike Bishop shows the different sword examples with the two bars.
The following is a reproduction made my Matt Lukes which shows the two bars on a Pompeii type Gladius
-painting by John Warry
Click on Pictures for higher resolution
(1) References to similar items: FEUGERE, Michel; Weapons of the Romans, page 111 & 114 2002.
(2) Reference to similar items: BISHOP, M.C & COULSTON, J.C.N; Roman Military Equipment "From the Punic wars to the Fall of Rome", page 81 2006.
**Note on background. Close up view of the wall of the Colosseum of Pula, Croatia. Picture taken 2014