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

The yellow leading edges on Mk. I's,Ia's and II often were full-span with individual variations as they were applied at MU and Sqn level.Mk. III's show variations,as well.
Most Mk. IV's were natural metal hence the difficulty in observing them on b&w photos.

The AirBritain series of serial number books contain several good photos of Mk. IV's with visible yellow leading edges.Quite a few of 213 Sqn Mustang IV photos show them,as well.

Stein