A Small Arm shall not be sold or exchanged, or be attempted to be sold or exchanged, or be exposed or kept for Sale or Exchange, or be exported or attempted to be exported or be kept for Exportation, unless and until the Barrel or every Barrel thereof has been duly proved at the Proof House or a Branch Proof House of either of the Two Companies, or some other public Proof House established by Law, and duly marked as proved.