The US Allison engined P-51s were not fitted in USAAF service with British 20mm cannon, but US manufactured 20mm cannon, produced by Oldsmobile to the basic Hispano 20mm cannon design. The US manufactured 20mm cannon were included in the P-51s supplied to the RAF, however the RAF armament trials and initial in service use identified a series of major issues with the US manufactured 20mm cannon, the NAA designed mounts and the US manufactured ammunition. As a result the RAF initiated a major program to fix the issues, resulting in replacement of all the US manufactured 20mm cannon in the RAF Mustang Mk.IAs with British manufactured Hispano Mk.II* 20mm cannon, revised ammunition loading, revised case and link ejection system and greatly beefed up cannon mounting. A visual sign of these changes being implemented, is on the USAAF P-51s, EXTERNAL recoil spring ahead of the wing cannon shroud, on the RAF Mustang Mk.IAs, INTERNAL recoil spring inside the wing cannon shroud and basically the bare cannon barrel and muzzle projecting forwards of the shroud.
Certainly there is much discussion and research gone into the Mustang, Apache and Invader names and their use relating to various variants of the P-51/A-36 family of airframes. Essentially, Apache was a NAA internal marketing name being considered as the name for the P-51/A-36 family of aircraft for sale to the USAAF. The RAF had selected the name Mustang for their aircraft some time earlier and to avoid confusion NAA dropped the use of Apache before it was ever considered for adoption. However the name did show up as result of early marketing material produced by NAA when they were trying to generate interest in the USA with the USAAF. The RAF use of the name Mustang stuck. Again, due to mis-communication between various parties the names Apache and Invader did pop up in various official documentation - as an example I have a copy of a US War Office issued aircraft recognition manual produced in early 1944, that shows a RAF Mustang Mk.I type aircraft silhouette, with aircraft name shown as Mustang, with an annotation about dive bomber variant also being known as 'Invader'. Presented with the documented evidence that the A-36 was never offfically named Apache in USAAF service, the USAF Museum has amended the story board in front of the A-36 on display in their collection to remove use of the Apache name, other than peripheral mention in the text regarding the confusion over the name. The official wartime USAAF/War Office list of aircraft types, designations and approved names, did include Mustang for the P-51/A-36 family, but did not include Apache or Invader.
The RAF removed the bomb racks that were fitted to their P-51As/Mustang Mk.IIs, as they were all allocated to use as low level tactical reconnaissance aircraft, their radius of action in the ETO on internal fuel was sufficient for RAF requirements, so no need for the racks and associated tanks. Taking them off gave a few more knots in airspeed. Late in WW2, some experiments were being conducted on RAF Mustang Mk.IIs on forward facing camera installations to accept large negative format cameras. This was to allow coverage of linear targets as an aircraft flew towards them, so flying along rivers, canals, roads, etc. Part of this experimentation included fitting 35mm cine cameras into modified drop tanks, hence the need for something once again to put the drop tank onto. Also produced great quality film footage, much better than standard gun camera footage, of low level strafing attacks.
RAF modified the Allison engines on their Mustang Mk.IIs similar to what they had done with the earlier engines in the Mustang Mk.I and Mk.IA, to produce better low down power and response, operating at 6,000ft and below. That was the prime operating range for the sorties that they flew, based around the camera and camera lense combinations they used in both the oblique and vertical camera mounts and what was considered to be the best range for visual acquisition and identification by the Tac/R pilots of targets of interest on the ground from the air.