Dear Jutocsa,
Since the WM 21 was a further development of the venerable Fokker C.V I took up the challenge and wrote a book about the WM 21 Solyom (
http://www.lanasta.eu/Shop/product/186/07-warplane.html). I don't know by heart if I placed cockpit photos but I'll check for you!
There are some interior photos in my books on the Fokker C.V!
Regards,
Edwin