[-)-MAILMAN-]

After posting this question I found this.

If you start the game and at the main menu select the objects radio button, then select aircraft and then allies and you scroll down to the the P-51B-NA and P-51C-NT then select the description you get this:

P-51B-NA
P-51B-1-NA, 1943
Engine: 1 x V-1650-3 Merlin
Take-off: 1,400 HP
1,450 HP At 6040m

P-51C-NT
P-51C-10-NT, 1943
Engine: 1 x V-1650-7 Merlin
Take-off: 1,450 HP
1,695 HP At 3,140m

If you look at the performance graphs in IL2 compare it appears that the the P-51C-NT has a slight performance advantage over the P-51B-NA.

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Scrolling down the object descriptions of the P-51B-NA and P-51C-NT you see listed some of the production changes by block:

P-51B-5-NA, 1943
Engine: 1 x V-1650-3 Merlin
Take-off: 1,400 HP
1,450 HP At 6040
additional 321-liter self-sealing fuel tank behind the pilot
The game's P-51B-NA has the 85 gallon fuselage tank.

P-51C-5-NT, 1943
Engine: 1 x V-1650-7 Merlin
Take-off: 1,450 HP
1,695 HP At 3,140m
additional 321-liter self-sealing fuel tank behind the pilot
The game's P-51C-NT has the 85 gallon fuselage tank.

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Online research I found additional facts:

P-51B-7-NA
added an additional 85 gallon self-sealing fuel tank behind the pilot
The gam'e P-51B-NA has the 85 gallon fuselage tank.

P-51C-3-NT
added an additional 85 gallon self-sealing fuel tank behind the pilot
The game's P-51C-NT has the 85 gallon fuselage tank.


P-51B-15-NA
Engine: 1 x V-1650-7 Merlin
Inglewood Production line made this engine standard with this block.

P-51C-5-NT
Engine: 1 x V-1650-7 Merlin
Dallas Production line made this engine standard with this block.
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RAF designated the variant Mustang-III. RAF designated both the B & C as Mustang III. British planes were different from the American versions mainly in the shape of the canopy. British Mustang had a teardrop canopy in place of a visibility-hampering faired cockpit. As the canopy was produced by P. Malcolm & Co, the new canopy was known as the Malcolm Hood. Many of the P-51B & C in the 8th & 9th AF were retrofitted with this Malcolm Hood.

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So in the game we have at least the P-51B-7-NA with the V-1650-3 Merlin and the 85 gallon fuselage tank behind the pilot & the P-51C-3-NT with the V-1650-3 Merlin and the 85 gallon fuselage tank behind the pilot and has been retrofitted with the Malcolm Hood.

Does the P-51B-NA represent the P-51B-7-NA with the Packard Merlin V-1650-3 and the 85 gallon fuel tank behind the pilot and the fared canopy or Does it represent the P-51B-15-NA (or later) with the Packard Merlin V-1650-7 and its modified supercharger which increased its low - medium performance at a cost of slightly decreased performance at high altitude compared to the earlier engine plus the 85 gallon fuel tank behind the pilot?

Does the P-51C-NT represent the P-51C-3-NT with the Packard Merlin V-1650-3 and the 85 gallon fuel tank behind the pilot and retrofitted Malcolm Hood or Does it represent the P-51C-5-NT (or later) with the Packard Merlin V-1650-7 and its modified supercharger which increased its low - medium performance at a cost of slightly decreased performance at high altitude compared to the earlier engine plus the 85 gallon fuel tank behind the pilot and retrofitted Malcolm Hood?

Mustang III, 1944
It has the 85 gallon fuel tank behind the pilot to go along with the retrofitted Malcolm Hood, but does it use the V-1650-3 or the V-1650-7? What else is different between the RAF version versus USAAF versions.