Hallo!
Marc-André Haldimann has two photos of JG25 Bf 109 G-5 W.Nr. 15 564 "CN+WH" "Schwarze K" at his site:
https://www.flickr.com/photos/280920...57640149616715
This might be the same "K" seen at the Estonian archive photo background.
BTW the archive does sell copies of the digital photos, at least they used to. Go to:
https://www.ra.ee/en/film-photo-audio/foto/ and see services. It has changed a lot from the latest time I used it.
Cheers,
Kari