Ok, I'll join the fun! As stated the B and C model are identical in appearance and both versions can be found with the flat canopy or the Malcolm hood. The dorsal fin was a field modification to some B and C Mustangs though I'm not sure why it was added. The D has the teardrop canopy and was introduced without a dorsal fin but one was quickly added to replace some of the side area (and stability) lost with the lowered rear fuselage. The B/C only had four .50s, two in each wing, while the D had six total. The leading edge strake (where the main wheel folds up into) on the D is much more pronounced than that of the B/C and the clamshell gear doors are slightly different as well. That's all I can think of, anybody got anymore?

Here is the Malcolm hood going on Jack Roush's P-51B restoration.