I think what may have happened is the middle .50 on the P-51D was moved from the secondary weapon to the primary.
Primary used to only fire the inboard pair only and secondary were the outboard pair or pairs, depending on ammo loadout. Now primary fires the two inboard pairs and the outboard pair is fired with secondary.
EDIT- further research shows the gun layout has changed.
P-51D
Take the 6 gun option, primary is the outboard 2x .50s, secondary the inboard 4x.50s.
Take the 4 gun option, primary is the outboard 2x.50s and the inboard 2x.50 become secondary guns. The middle pair are visible of course but are not loaded.
Mentioned here:
http://www.flyaceshigh.com/forums/showthread.php?s=&threadid=180607