In 1914 the British experimented with maps by imposing their own grid lines of 1,000 yard 'artillery' squares on reproductions of Belgian maps which were created along metric sheet lines. This resulted in an improper fit of the grid to the map and created problems when trying to plot targets over 2 or more map sheets as the grids overlapped rather than aligned with adjacent sheets. This experiment was called the "B" series and was later dropped.

The image below was taken from a map in our collection (ID #100ww1map).
Notice how the grid extends upwards past the edge of the base map at the top, and the last column of squares along the right edge is cut off at the halfway point.