off fairings; the single-stage supercharger of the Griffon II or IV used in the Mk XIIs meant that it was rated and used as a low altitude fighter and the LF prefix used by Merlin-powered Spitfires was never applied. Starting in early 1945 most Spitfire Mk XIVs also used clipped wing tips, mainly to reduce wrinkling of the wing's skin; again the LF prefix was not applied to these aircraft. Redesigned late wing As the Spitfire gained more power and was able to fly at greater speeds the risk of aileron reversal was increasing so the Supermarine design team set about redesigning the wings to counter this possibility. The original wing design had a theoretical aileron-reversal speed of , which was somewhat lower than that of some contemporary fighters. The new wing of the Spitfire F Mk 21 and its successors was designed to help alleviate this problem; the wing's stiffness was increased by 47 per cent and a new design of aileron using piano hinges and geared trim tabs meant the theoretical aileron-reversal speed was increased to . This wing entered service on the Spitfire XXI. The standard armament was now four 20mm Hispano IIs or the shorter, lighter Hispano V cannons, each with 150 rounds per gun. Variants Notes The Mark numbers used in the aircraft designations did not necessarily indicate a chronological order; for example, the Mk IX was a stopgap measure brought into production before the Mks VII and VIII. Some Spitfires of one mark or variant may have been modified to another; for example, several of the first Mk Vbs were converted from Mk Ibs; the first Mk IXs were originally Mk Vcs. Up until the end of 1942, the RAF always used Roman numerals for mark numbers. From 1943–1948, new aircraft entering service were given Arabic numerals for mark numbers but older aircraft retained their Roman numerals. From 1948, Arabic numerals were used exclusively. Thus, the Spitfire PR Mk XIX became the PR 19 after 1948. This article adopts the convention of using Roman numerals for the Mks I–XX and Arabic numerals for the Mks 21–24. Type numbers (such as type 361) are the drawing board design numbers allocated by Supermarine. Mk IV / XX (type 337) On 4 December 1939, the Supermarine design staff produced a brochure which mooted the idea of converting the Spitfire to use the Rolls-Royce Griffon engine. A top