This is the best description I've found, it gives the full description of Gustav's post above
http://www.german-navy.de/kriegsmari.../feature6.html
So in the square shown in Gustav's post that woul be AN (or whatever) 5619 - if it was the middle mini square highlighted (the red 1), it would be 5615.... and so on...
Each of those mini squares are 6 nautical miles by 6 nautical miles.