Mk. IX history snipit...
In response to the introduction of the Fw-190, another 'interim' mark of Spitfire was proposed, pending full scale development of the Mk VIII. The result was the Mk IX. Like the Mk V, this 'stop-gap' was also an outstanding success, 5,665 being built, the second highest number of any mark!
The Mk.IX series was powered by the Rolls-Royce Merlin 61, 12 cylinder V, liquid-cooled, generating 1,565 hp.
It was developed in response to the appearance of the Focke-Wulf 190 that, in 1942, was decided superior to the Mk.V in everything except turn radius.
The Mk.IV was basically the Mk.V airframe married to the more powerful Merlin engine.
A total 5,665 were built in three basic sub-series designed for low, medium and high altitude combat (with the LF, F and HF wings) and armament type (b, c or e) b's and c's had four 0.303 machine guns and two 20 mm cannons, and e's had two 12.7 mm machine guns and two cannons and up to a 1,002 lb (454 kg) bomb load.
Numerous subvariants of the Mk.IX were produced and given other numbers in the series.