The Forge's Word
The steel-boss shield is the Iron-Boss Shield's argument finished by the forge. The boss is the dome that stands proud of the face, and the whole difference fits in one word: the boss is steel. The iron boss takes the blow and gives the blow back the way iron does — into the rivets, into the hand, into the wrist. The steel boss takes the blow and keeps the blow — the edge of the blow dies in the steel, and the steel does not tell the hand about it.
A master steel-boss shield is an oak face — or a face of Hide over a core of Bone for the lighter work — with the boss forged from Steel and set with a ring of rivets, because the boss is the part that moves and the rivets are what let it move and come back. A masterwork steel-boss shield is a story event, and the story has not written it yet, because the masterwork is not the kind of shield a story keeps in its pocket.
In the Hand
The steel-boss shield is the third stance, made steel at the center: brace. The fighter aims the boss at the blow, and the boss takes it, and the hand takes what the steel does not give — which is the least. The steel boss is the last word of the forge, and the forge's last word is short: not you, you are behind me, and I am the one who is hit.
"Iron tells your wrist about the blow. Steel does not tell. That is the difference between a shield and a wall." — Tigole