[Snautzer]

thanks

i haven't seen them on later mark spitfires. indeed with only cannon there arent any.

So thats why i thing that the .303 mg in the earlier mark needed a vent.
(Its obvious why the VA and VB had them as well. mark V being based on the airframe from a mrk I/II with a merlin 45 engine )
With less .303 on board the become scarser.

Heating air to the guns possible . air passing the vent drawing air out of the gun bays. hot air drawn from engine. I seem to remember about electric heating for the guns though