The "boss of a shield"  is called a buckle, from the Middle English "bocle" and the Old French "bocle" or "bucle" . The "boss of the shield" is the ring of the shield. Source: Webster's New Twentieth Century Dictionary of the English Language. 1943 

 The "boss of the shield" in Late Latin is called a "bucula' or "buccula",or the beaver of a helmet from the bulging shape. Source: An Anglo-Saxon Dictionary by T. Northcote Toller (1921)cited @ lexicon.ff.cuni.cz/html/oe_bosworthtoller/d0110.html