For the battle at Arnhem (Market Garden) there are several War Diaries available. In these there are mentions of coordinates. The style is 697774 for example. These are based on US Army maps available at the time. 69 is from west to east, 77 is from north to south but the last two are unclear to me. Would these be altitude? I cannot place it.. I've uploaded a screenshot of an entry in a war diary and a simple render based on the US Army map. Sorry for the potato quality of the war diary. it was sent to me like this. A picture of a monitor with a screenshot. Album
The third digit is a way of specifying specific points within a particular grid square. It could equivalently be represented as 69.7,77.4 - i.e., the point specified is seven tenths of the way between 69 and 70, and four tenths of the distance between 77 and 78. Divide each side of the grid square in question into tenths, and you should be able to read things off as needed. These lines are not overlaid on the map because to do so would obliterate much of the geographic detail.