An all-metal, stressed-skin wing of duraluminium (a DERD specification similar to AA2024) was introduced in April 1939 and was used for all of the later marks. "The metal skinned wings allowed a diving speed that was higher than the fabric-covered ones. They were very different in construction but were interchangeable with the fabric-covered wings; one trials Hurricane, "L1877", was even flown with a fabric-covered port wing and metal-covered starboard wing. The great advantage of the metal-covered wings over the fabric ones was that the metal ones could carry far greater stress loads without needing so much structure beneath." Several fabric-wing Hurricanes were still in service during the Battle of Britain, although a good number had had their wings replaced during servicing or after repair. Changing the wings required only three hours work per aircraft.