There have been three versions of A2A P-51 released. The first version (the FS9 P-51D/H pack) is for FS2004. The second version (Wings of Power II/ World War II Fighters) was kind of an upgraded version, right around when FSX was released, so it was for both versions of Flight Simulator. The final version, the Wings of Power III P-51D and/or Civilian Mustang for Accusim, are not compatible at all with FS2004.
The new Aircraft Factory P-51H is also not compatible with FS2004.
I hope this clarifies.
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