fd ski:
From The Great Book of World War II Airplanes: "Most of the 5,600 Spitfire IXs were built as LF.IXs, their supercharger gearing optimised for best performance below 6,000 feet (1,830m); less than a quarter had the medium-altitude-rated Merlin, and a tenth of the IXs built were HF models with engines rated at 22,500ft (6,850m) in top gear."
The least produced IX was the HF.IX.
From Jane's All the World's Aircraft, 1945/6 Edition, it is stated that the F, LF and HF models of the IX "May be fitted with 'E' armament".
It would seem that Pyro's choice of F.IX wasn't all that unwarranted.
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129 IAP VVS RKKA