Mikkel,
What they are referring to is the Map Grid Reference system where they refer to the letter and series of numbers. See the website below for a description of how the system worked and a coordinates translator that gives a fairly close translation of a set of grid coordinates into a location.
http://www.echodelta.net/mbs/eng-welcome.php
For the notation as to what areas 'B and C' were you would have to find a description somewhere in Squadron records, Wing records or hopefully recorded in a pilot's log book of what those exact areas were. I know for Tac/R pilots, areas of the Netherlands were designated by an area title that represented a reconnaissance area to be covered in a sortie - almost like a bus route.
Hope this helps.
Regards,