Thanks Nick
With ref to Barry Rosch I am led to believe he has relied quite heavily on books published about the German Feld Post, how it worked and how its codes corresponded to various flying Luftwaffe units. Personally I have not a clue how it worked

There is NO clue in Rosch's book if he has used ANY primary sources at all....
Also it would not surprise me one bit if Mikael Holm has glanced a bit in Rosch's book as well. Don't know this of course....
Cheers
Stig