WONDERING if i should even do this.....but i HAVE to...........
The British had run low on cash, and so on 7 July 1941, the USAAC had ordered 150 "NA-91s" for them, which were formally designated P-51. This was before the RAF had even received its first evaluation Mustang. This Lend-Lease batch of aircraft had an unusual weapons fit, with gun armament consisting of four Hispano-Suiza Mark II 20 millimeter cannon, replacing the six wing-mounted machine guns.
The cannon armament was introduced at the request of the British, who had been plagued during the Battle of Britain with the feeble firepower of Hurricanes and Spitfires armed solely with banks of rifle-caliber 7.9 millimeter (0.303 caliber) machine guns. The Mustang's two nose machine guns were deleted, and the three machine guns in each wing were replaced by two cannon. The Hispano Mark II had been designed as a "motorcannon", fitting over the cylinder banks of an inline vee engine and with the muzzle sticking out the prop spinner. This meant the weapon was very long and its muzzles distinctively protruded well out in front of the wing of the P-51. Despite the hitting power of this heavy armament, for whatever reasons no other major model of Mustang would feature it.
http://www.vectorsite.net/avp51_1.html#m1