I was looking at the Spitfire cockpit graphics again when I noticed that the trigger in the Mk IX and Mk V cockpits was for a Spitfire armed only with machine guns such as a Spitfire Mk Ia or Mk Va.
In this screenshot you can see the trigger button is a simple, single action device used to fire all weapons:

Here is a photo of a Spitfire Mk IX's trigger mechanism:

Pressing the top fired the machine guns and pressing the bottom fired the cannons.
Here is a second shot of it:

Pressing the middle fired all guns.
And here is a shot of the action used in controling the ailerons:
