Brian
This topic has been up on these pages before, and I even think you were the one...
I have nothing to add, except that previously the code for the captured Do 18 was stated as K6+YK! However checking K6 and M7 none of these prefixes fits Kü.Fl.Gr.506 which had S4
K6 was Kü.Fl.Gr.306 and M7 was Kü.Fl.Gr.806. That said I also notice the shovelling around of their various staffeln between each other in all these naval units and I find it more or less a miracle that anyone in end knew to which unit he belonged...
Cheers
Stig