Historical
BS EN ISO 4032:2012
Hexagon regular nuts (style 1). Product grades A and B (British Standard)
ISO 4032:2012 specifies the characteristics of hexagon regular nuts (style 1), with threads from M1,6 up to and including M64, with product grade A for threads of diameter less than or equal to M16 and product grade B for threads of diameter greater than M16.
British Standards Institution [bsi]