The P-51 B (Mustang Mk.III if you prefer) has been acurrately modeled?

[Vierzinger]

Sources:
"P-51 Mustang"
- William Newby Grant
"The Encyclopedia of Aircraft of WWIII"
- Paul Eden

First of all. The mustang on your picture are a P-51-NA, Allison engine and with 4x20mm. The US ordered "...150 Mustang IAs with their quadruple 20mm M-2 cannons as P-51-NAs in September 1941,...." - William Newby Grant
I guess that explains the US markings on a british model.

Same as the British Mk. IA.
BTW, the early IA had 2x.50in in each wing and .3in two mounted below the engine.

The A-36A "Apache"
did not have 20mm cannons but two .5in MG in each wing.
The where Allison engined as well.

But the real question are the difference in IL-2, between the Mustang Mk. III and the P-51 B/C models.

First of all, the B/C does not differ.
It is a stupid american distinguishing between the plane build on different factories. It was later changed.

In the game, the "C" version have a british Malcom hood. This was not standard. But a retrofit which only happende at some USAF sqd based in England. If the maintenace crews could get it from the british.

The Mustang Mk. III and the P-51C should be identical in the game. Both are merlin engined, but I think I have read somewhere that the Mk.III are a suped up (tuned) version.
Which is historical but where only done if they sqd's in question was assigned to intercept V-1 rockets (cruisemissiles).