They had 2 .50cals under the nose. as well. From the earlist version I thought they always had this. Later it was removed in the P-51B because of difficulties accessing them and reduced space due to the new engine setup.
It's also one of the reasons the Italians dropped them on the G.55 -- too hard to service and get to under there.
The P-51A only had 4x .50 calibers.
Model - P-51A
Production - 310
Length - 32.25
Height - 12.2
Wingspan - 37.04
Weight - empty - 6433
Weight - normal T.O. - 8600
Weight - max G.W. - 10600
Powerplant - Allison V-1710-81
Horsepower - 1200
Propeller - Curtiss 3-bladed electric 10'9"
Max Speed - 390 @ 20k
Service Ceiling - 31,350
Fuel Capacity - 180
Drop Tanks - 2x 75 gal
Range - 750 / 1375+
Guns - 4x .50 cal - 1260 rounds Bomb / Rockets - 2x 500 lb bombs
I think the confusion lies with the designation that the USAAF used for the Mustang I (P-51), which leads a lot to think that this was the first fighter production model of the Mustang for the USAAF, when none of the P-51s saw operational service save for those that were converted to tactical recce planes (F-6A). As I mentioned already, the first fighter production model for the USAAF was the P-51A, which in the RAF service was the Mustang II. The P-51A had the .303 caliber machine guns removed as well as the two nose mounted .50 cals and added two more .50 cals to the wings to give the P-51A 4x .50 cals.
ack-ack