noun
The primary modern meaning is 'sheet metal' (metal rolled or hammered into thin sheets). The 'dagger' or 'sword' sense is archaic and refers to a blade made from such metal, now mostly encountered in historical contexts.
工場で延べ金を切断する。
They cut sheet metal at the factory.
古文書には延べ金で作られた短剣の記述がある。
Ancient documents describe a dagger made of sheet metal.
板金 is the more common modern term for sheet metal, especially in industrial contexts. 延べ金 is less frequent and carries the additional archaic blade meaning.
延べ (のべ, 'stretching, flattening') + 金 (かね/がね, 'metal'). Originally referred to metal that has been hammered or rolled into a thin sheet, and by extension a blade made from such metal.