Great stuff Paul. The pic isn't showing, just a 'J.Peg' icon.
The RAF Mitchells had the U.S. serial painted over in most cases, and this normally showed as a darker rectangle on the fin and rudder, probably done at MUs in the UK, using British paints. Others very well may have had them over-sprayed in O/D. at the factory after allocation to the RAF.
See the example on my subject, below, which also appears to have a darker area where it's possible that the 'Star and Bar' marking was over-painted, on the fuselage, although this could be fresh paint where the codes were changed or moved, along with the roundel.
I think I'm probably OK now for most things on the Mitchell MkII (B-25C and D), but if you can confirm the hyphen in the British serial number was only on Kansas produced aircraft, that would be helpful, and any close-up, or relatively close-up pics of the antenna wires would be useful too.
And anything you have in the way of photos of RAF Mitchell MkIIIs (B-25J) would be most useful - decent shots are fairly rare.
Remind me which book it is, as I might end up getting it, seeing as I have a Mitchell MkIII to build yet !