Hi Norbert
I’m afraid that your ice has broken!
For the post 1939 period the normal code system is that the third character is the a/c identification letter and the fourth character is the staffel identifier. Not all letters were used, and the letters A to F were for stab(staff flights). Staffel identifiers began then with the letter H.
For your example +FK this would be aircraft ‘F’ of the 2nd staffel.
There is no gruppe identification explicit in the code, although there had been in the pre-1939 system. There are a couple of units which used different coding systems, but as far as I know none of these used Ju88s. A few other unit types ie fighters and (some) ground attack, did of course use other systems.
Ian