Thanks for the responses chaps, it’s certainly a bit of a conundrum. As stated above, I haven’t seen this fit on any other spitfire. (apart from one shot of K5054, in camouflage, which I cant find at the moment, but it’s in “Spitfire the History” by Price.)
I don’t think this was a common modification, and It may even have been a one off. More digging required!

Cheers again,

Mark