Symbols on a 1950's OS map?

[harrym1byt]

I am looking at an old (1950's) OS map of Garforth. http://maps.nls.uk/view/100947119

Some of the buildings (houses) are marked with a simple box outline, others which are similar are marked with a box which is hatched internally, yet others are marked with a very lengthy box representing several houses, which are semi's.

Does anyone know what the differences in the markings on the map might be please?

[warringtonrhino]

a solid block indicates a public or important building
shapes with diagonal (single) hatching are dwellings etc
shapes with cross hatching are glazed buildings
shapes with no hatching are buildings with no roof, either roofless relics or buildings proposed or half built when the survey was done.