There shall not be kept within the city more than fifty (50) pounds of powder, or other explosive substance, by any one person, and every person keeping such articles shall keep all they have in one metal case, with metal cover and handles, plainly marked "POWDER," which shall be kept near the front door, and shall display conspicuously upon the front of the building a sign plainly marked "GUNPOWDER KEPT HERE." (Prior code § 26.01.018)