Interesting - I read a book by a local (Ottawa area) writer who flew 'Mustang III' aircraft in low level RAF recon unit. He mentions armament of 4x20mm guns, which I thought must have been a mistake of memory on his part. His engine was optimized for fastest low altitude flight. He operated from base in Britain. He was also sent on solo raids to attack low altitude aircraft and shot down some bombers with those 20mms.
Thanks for backing him up. I will have to dig out book to remember his name, unit, etc., but it seems that the RAF must wound up with some of those p51Bs you mention.