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