@Giorgio N
Thought it might be best to move further discussion on converting the new-tool Airfix VC into a Srafire II or III here, so as to not clutter @tonyot's excellent WIP and steal his well-deserved thunder. I pulled my Valiant Wings Spitfire monograph from my library, (I had forgotten it included Seafires!) and here are some observations based on the content:
It appears in the diagrams and photos, that the 20mm cannon was mounted in the inner bay, for the versions with either the wide or narrow bulge. The stub where the outer cannon would have been mounted was r