The National Grid Series was introduced after the Second World War and superceded the County Series.
The scale of 1:2500 is 25.344 inches to the mile. These maps are sometimes referred to as 25 inch maps.
The maps were produced either as a single 1km square (i.e. a single grid square on a modern 1:50000 Landranger) or, more commonly (at least in our collection) as a pair of adjacent 1km squares.