An Iron shield boss/umbo, believed to be for a Roman or Celtic shield.

This example is corroded, yet retains its form well, clearly showing one of the rivet holes where the umbo would have attached to the shield.  The Shield boss provided an opening for the hand to protect it, as well as a solid area for knocking into the enemy. A handle would normally have ran on the inside in a horizontal direction, allowing for the left hand to hold the shield while the right carried the sword/gladius.

The boss/umbo would have attached to the front of a flat Oval or Hexagonal shield used by both the Cavalry and Auxiliary units.  The legionary generally had a curved rectangular shield during Rome's peak, allowing for better protection. The shield umbo for that shield was also much larger and had a square shape for attachment. There is evidence that Legionaries and Auxiliaries alike had flat oval shields especially in the later empire when there was far less distinction between the two units. So this umbo could have been used by both a Legionary and an Auxiliary within the Roman army. (1)(2)

-painting by M. Daniels

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Reverse view Side view Edge view Museum shield boss example  approximately 20cm wide