Hello Raymic,
I can't make out what is there in the other photos but the clear one of U 552 shows a vertical pole which looks like it is attached to the MG34. The pole could be removable rather than permanently attached. It would be awkward to take them and store them in a hatch within the main deck, though this could be possible. Maybe they could have stored them within the boat as getting them down the tower hatch would be possible?
Cheers,
Dougie