No, you're not misinformed, just not fully informed. Saying a Spitfire is a "Spitfire Mk IX" tells us just as much as saying a Bf109 is a "Bf109G". It narrows it down some, but not nearly to specifics.
The bottom line here is that we have a Spitfire F.Mk IX powered by a Merlin 61 engine and equiped with a Universal wing. This is the Spitfire Mk IX that entered service in July, 1942. The Universal winged Spitfire could, technically, take eight .303s or two 20mm cannon and four .303s or four 20mm cannon. In practice it always carried two 20mm cannon and four .303s.
The "e" wing which had two 20mm cannons and two .50s doesn't show up until 1944 on the Spitfire LF.Mk IXe powered by the Merlin 66 (99.995% the same airplane as the Spitfire Mk XVI we have) and Spitfire HF.Mk IXe powered by the Merlin 70.
The Spitfire Mk IX in AH covers the initial introduction of two stage, two gear Merlins to Spitfires, the Mk VIII covers the next stage of it and the Mk XVI covers the peak Merlin Spits.