sorry guppy you  are in correct on the RAF mustangs and I will get the details soon . can't get to the book right now (i'm not at home ) but the RAF did get cannon armed mustangs . It was not a complete order as the attack on pearl harbour  forced to cancellation of the order on the US side .  These  became USAAF  armed recon aircraft . Not the A36 or known as the MkIa . But the 100 odd RAF P-51 MkIa  were cannon armed with another 200 with the .50cal delivered  before the pearl harbour attack.   I can give you Squadrons  that they equipped if you'd like  and service dates. 
Not questioning that they got some cannon birds.  They were not the original order for the RAF.  Those were MG birds.  We've had this discussion, complete with photos of both kinds in RAF service previously 

Of the Allison Mustangs, the connon bird is the least important in terms of service use and numbers is all I'm saying.  The RAF Mustangs over Dieppe for example in 42 were MG equipped.  The CBI first Air Commando Allison Mustangs were MG birds.  The A36 was far more important, and an MG bird.
Folks want the cannon bird, not because it's a Mustang, but because they see 4 cannons and try and make the argument it's because it's a Mustang 
