Yes, this thread HAS been derailed.
Well Pongo posted a reply (c page 6 midaway down) using a quote of mine stating that the first mustangs
where cannon version (Mustang1As) when in fact they were Mustang1s with a weapons
package similar to the P40B...
Just for the record that is incorrect, the early production variants of the P-51 were XP-51, P-51, A-36, P-51A.
I believe the prototype XP-51 was unarmed, and according to Dean (America's Hundred Thousand) The next model was the P-51 which had four 20mm cannons, from there they went to the A-36 which in addition to other changes had the armament changed to 4 .50's in the wings and 2 under the nose. The next model the P-51A changed it again to the 4 .50 cal machine guns in the wings that we see on the B and C models.
*edit*
I was wrong myself on the XP-51/NA-73X armament, it is not mentioned in Dean, but online sources do have it listed as 4 .30's and 2 .50's cal MG's. So as with most arguments it comes down to semantics. The prototypes did have a similar armament to the P-40B but the first production models that were delivered to units did not, at least as near as I can figure.