the Mk VI, the Mk VII was a high altitude pressurised variant, this time powered by the Merlin 64 (F. Mk. VII) or 71 (H.F. Mk. VII) series engine with two-stage, two-speed superchargers. The cockpit was pressurised in a similar way to that of the Mk VI, although the details were slightly different. Other changes to the airframe are noted above. The Mk VII used a Marshall manufactured compressor for pressurising the cockpit; this was mounted on the right of the engine and drew its air through a long intake under the starboard exhaust stubs. An automatic valve allowed a maximum pressure differential of +2 lb./sq.in. This was built up during the climb and was maintained at heights of 28,000 ft and above. Extended, "pointed" wingtips were fitted to the Type C wings, increasing the wingspan to 40 ft 2 in (12.2 m). Because the threat from high altitude bombers never materialised many Mk VIIs later reverted to the normal, rounded wingtip. While early Mk VIIs were fitted with a detachable canopy, secured by four pilot-operated catches, later Mk VIIs were fitted with a "Lobelle"-type hood which opened by sliding backwards, as on non-pressurised versions of the Spitfire. This was a big improvement on the clampdown cockpit of the Mk VI. The canopy was double-glazed and used rubber tubing to create a proper pressure seal against the fuselage. The canopy rails were bulkier than the standard Spitfire type. In total, 140 Mk VIIs were built by Supermarine. Most of them were powered by the Merlin 64 (F. Mk. VII) or Merlin 71 (H.F Mk. VII), the latter fitted with a Bendix-Stromberg "anti-g" carburettor. The HF Mk had superb high-altitude performance, with a service ceiling of ; French ace Pierre Clostermann recalls in his book, The Big Show, the successful interception of a long-range reconnaissance Messerschmitt Bf 109G-6/R3 by a Mk VII 'Strato Spitfire' of 602 Squadron at over the British Home Fleet's base at Scapa Flow in early 1944. Mk VIII (type 360) The Mk VIII was an adaptation of the Mk VII without the pressurised cabin and was intended to become the main production model of the Spitfire. When the "interim" Mk IX proved to be adequate against the German Focke Wulf 190 it was decided to use the shadow factory at Castle Bromwich to produce that version only; the Mk VIII Spitfires were all built by Supermarine. Apart