ŝildo

The two meanings look unrelated in English and are one word in German, and Esperanto simply inherited the arrangement. Der Schild is the thing you hold up in a fight; das Schild is the board with writing on it. German keeps them apart by grammatical gender, which Esperanto does not have, so here they are frankly the same noun and context does the work. In practice this causes no trouble at all: nobody has ever had to guess whether a shop was defending itself.

There is no seam to find. Ŝild- is one root; the language contains no ŝildro or any other near-neighbour to confuse it with, and the only thing that regularly trips people is the initial ŝ, which the x-system writes sx and which is the same sound as the English sh.

Both senses build normally. Ŝildi is to shield or to screen something, and heraldry - a field with a great deal of Esperanto vocabulary attached to it - calls the coat of arms a blazono on its ŝildo. For the plain sign outside a door, ŝildo and tabulo overlap: a tabulo is the board as an object, a ŝildo is a board that is telling you something.