While I agree with the others on pennant and letters I'm not sure if I'd dismiss the all-yellow leading edges...
It seems to me quite a few RAF Mustangs had the yellow extended all the way to the wing fairing. I've done a quick search and can't find any IV's up front but several camouflaged III's show that it was quite common. Could be exclusive to the Mustang III though.
Here's an example:
http://www.frenkenstein.com/ww2/greece/Vass/Vass.htm
Regards,
Kyrre