Although these "clipped-wing" aircraft are popularly known as "L.F." versions due to the fact that they were designated "L.F." (i.e. "Spitfire L.F. Mk V"), the "L" actually refers to the different versions of the Rolls-Royce Merlin engines used, which were optimized for low-altitude performance with "cropped" supercharger impellers (Merlin 45M, 50M or 55M). While many "L.F" Spitfires were given "clipped" wings to accompany the new engine variants, a number still retained the original wingtips. The "A-type" was the original wing design, with eight .303 calibre Browning machine guns, with 300 rounds per gun (rpg). The basic structure of the wing was unchanged until the C type in 1942. The one major alteration made to this wing, soon after production started, was the incorporation of a heating system for the gun bays, to prevent the guns from freezing up at high altitudes. To achieve this, open structures, such as wing ribs, surrounding the gun-bays were closed off, forming closed bays for each gun. Ducting was added, which drew heated air from the engine radiator and transferred it into the now-closed weapon-bays. Underwing vents, covered by streamlined triangular blisters just inboard of the wingtips, extracted the air, creating a negative pressure differential, and caused more heated air to be drawn in, ensuring a steady supply of heated air without any need for a mechanical blower. To keep cold air from blowing in via the muzzle openings in the leading edge, they were sealed with red fabric adhesive tape by ground crew while loading the ammunition trays. The guns fired through the tape, so they were no longer sealed after combat. Towards the end of 1940, the fabric covered ailerons were replaced by ones covered in light alloy. The A-type wing was only compatible with the 8 × .303 Browning machine gun armament suite. The "B-type" wing was structurally an A type modified to carry one 20 mm Hispano cannon per wing, which replaced the two inner .303 machine guns. The retractable underwing landing lamp was repositioned. The cannon was installed in the innermost machine gun bay, next to the wheelwell, while the second gun was deleted altogether. The area where the inner guns' ammunition trays had been located, outboard of the cannon bay, was converted into a compartment to accommodate the 60-round drum magazine for the cannon.