Aluminum, will also apply on fittings or accessorial articles for installation not exceeding 10 percent of the weight upon which charges are assessed, not less than six sheets may be accepted in bundles, wired, strapped or otherwise prepared so as to securely hold sheets of roofing or siding together, applies whether or not corrugated, crimped, channeled or ribbed, but not when otherwise curved, bent or formed, in boxes or crates.