Franck,
This pic might be a good lead for understanding the particular "Schwarze 9" situation:
http://www.flickr.com/photos/2809206...n/photostream/
There you can see graphically illustrated both JG 4 Fw 190 A-7's and a Bf 109 G of IV./JG 3. Both units, were stationed at Salzwedel between 29 February and 6 June 1944. An easy explanation for the borrowing by JG 4 of a replacement engine cowling from a JG 3 machine?
Cheers
Marc