Hello Anders,
I am not sure what the difference between A and B is, though it is certainly to do with the quick diving hull (also known as the cut out foredeck). Only U 539 is known to have B. There is only one image of U 539 I have seen and that only shows a little part of the cut out foredeck.
U 804 was fitted with A between October 1944 and sometime in 1945.
The photo below is thought to be U 804. The yellow training band on the tower means it was taken during the training period in early 1944. The rust streaks means it must be at least a month or two after the commissioning ceremony (04/12/43) because boats were all freshly painted for commissioning. Note that the cut out foredeck has not yet been fitted. Here is the photo -
There is another image which is thought to be of U 804 (though I would not like to say for 100% that is is U 804) -
I don't know the source of the images or where the originals may be found unfortunately.
Note the problems with paint adhesion on the dark grey paint of the tower. This was a common problem with late war IXs seen on many other boats such as U 505, U 805 etc.
If you do not find any more photos, then you could look to U 805 as a reference for the details. Just because they were sister boats does not mean the modifications they received would be the same. But without photos to go for, then you could look to U 805.
Cheers,
Dougie