[carlmu]

Fernando!
The image you refer to is one of those taken in Pori 08.08.1941. The third letter you mean is C, is in fact not letter no 3 but no 4. I have got hold of this photo as high resolution TIF file, and when I blow it up, the letter no 3, which in fact is (VOILA!) K pops up. Very faint, but visible. I can't understand why this hasn't been seen before. Maybe because the letter stands little out from the camouflage paint. If you are interested, I can blow up the picture and cut out the Balkenkreuz with the the two last letter and post it here.
With this I think it is proven that Erich Groths plane was coded M8+KC