Machinery and tools, including repair and replacement parts, designed and used directly in manufacturing or processing materials, components, or equipment for national defense are hereby declared to be a separate class of property and shall constitute a classification for local taxation separate from other classifications of machinery and tools as defined in § 58.1-3507. The governing body of any county, city, or town may levy a tax on such machinery and tools at a different rate from that levied on other machinery and tools. The rate of tax imposed by the county, city, or town on such machinery and tools shall not exceed that applicable to the general class of machinery and tools.