Hmmm, I guess you're right. Where can i find an official reference to the symbol for Harmonium though? There's none on the Coppermind page for it.
Still, don't the first two you mention seem a little bit out of place? A Kredik Shaw logo and an Aon (even though the Ire are a big part of the book) among allomantic metals? Besides, we don't really know if the Kredik Shaw logo is actually just a symbol for the palace itself or it's actually a representation of a metal, do we?